Allergy & Immunology for GPs: Your Essential Guide
From sniffles to anaphylaxis — because ‘just antihistamines’ isn’t always the answer
Last Updated: 2026-03-22 — Sources verified against NICE CKS, RCUK 2021, BSACI, UKHSA Green Book
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🌐 Web Resources
- NICE CKS: Anaphylaxis
Emergency management and adrenaline prescribing guidance
- NICE CKS: Cow’s Milk Protein Allergy in Children
Formula pathway, diagnosis and management
- British Society for Allergy & Clinical Immunology
Clinical guidelines and patient resources
- Resuscitation Council UK: Anaphylaxis Algorithm 2021
Updated 2021 emergency treatment guidelines
- UKHSA Green Book: Immunisation Against Infectious Disease
Vaccines in allergic patients and contraindications
- Primary Care Dermatology Society – Allergy & Urticaria
Practical guideline with visuals — stepwise treatment, photos of presentations, prescribing guidance
- Allergy UK – Healthcare Professionals Section
Patient and clinician resources — leaflets, allergy vs intolerance explained, practical avoidance advice
🆕 High-Yield Teaching / Exam Prep
- Zero to Finals – Allergy & Anaphylaxis
Visual learning & exam prep — simple explanations, memorable structure, good for SCA + AKT
- TeachMePhysiology – Hypersensitivity Reactions
Visual pathophysiology — Type I–IV clearly explained with diagrams, summaries, and clinical relevance
- Southampton Allergy Centre – GP Resources
Trusted NHS guidance — food allergy pathways, testing guidance, practical case advice
🔬 Testing, Interpretation & Pitfalls
- North West Allergy Network – GP Education Resources
GP teaching resource — when NOT to test, IgE interpretation, referral criteria
- Royal United Hospitals Bath – Allergy Testing Guidance
Practical testing guide — skin prick vs blood tests, false positives explained, when to refer
💬 Patient Communication & Consultation Support
- Anaphylaxis UK – GP Resources
Patient safety & education — adrenaline auto-injector guidance, action plans, risk communication tools
- NHS – Food Allergy vs Intolerance
Patient explanation tool — clear allergy vs intolerance distinction, symptom timelines, reassurance scripts
An allergic reaction happens when the immune system reacts to something that is usually harmless — food, medicine, pollen, latex, or an insect sting. NICE defines drug allergy as a drug reaction with clinical features compatible with an immunological mechanism. Importantly, a patient can become allergic to something even if they were fine with it before. Prior tolerance does NOT rule out later allergy. (Source: NICE CKS; BSACI)
| Type | Mechanism | Timing | Typical Symptoms | Useful Tests? |
|---|---|---|---|---|
| 1. Immediate (IgE-mediated) | Specific IgE antibodies → mast cell degranulation → histamine release | Minutes to 2 hours | Urticaria, angioedema, itch, vomiting, wheeze, rhinitis, anaphylaxis | Yes — skin prick test, specific IgE blood test (if history fits) |
| 2. Delayed (non-IgE-mediated) | T-cell mediated (Type IV); not driven by IgE | Hours to days (cow’s milk NICE CKS: 2–72 hours) | Vomiting, diarrhoea, constipation, blood/mucus in stool, reflux, eczema, feed refusal, faltering growth (infants) | No — SPT and specific IgE often negative. Diagnose by elimination + reintroduction |
| 3. Mixed (IgE + non-IgE) | Features of both immediate and delayed reactions | Rapid urticaria/vomiting AND ongoing eczema/GI symptoms | Particularly relevant in cow’s milk allergy and some childhood food allergies | Targeted (for IgE component); clinical diagnosis for non-IgE component |
| 4. Non-allergic adverse reactions | NOT immune-mediated; intolerance, side effects, pharmacological, or irritant | Variable | GI symptoms (e.g., lactose intolerance = enzyme deficiency, not immune). Drug side effects (e.g., GI upset with erythromycin) | No allergy tests applicable. Clinical diagnosis. |
🔑 Early vs Late Reaction: A Practical Shortcut
Early reaction (minutes to 2 hours) = usually IgE-mediated — skin prick testing and specific IgE blood tests may be useful when the history identifies a likely allergen.
Late reaction (hours to days) = usually non-IgE-mediated — worked out from clinical pattern plus elimination-and-rechallenge. Skin prick tests are usually unhelpful here.
Patients (and sometimes clinicians) use these terms interchangeably. They are clinically distinct — and mixing them up causes real harm, from unnecessary food exclusion to missed true allergy.
| Term | Definition | Immune-mediated? | Life-threatening? | GP Action |
|---|---|---|---|---|
| Allergy | Clinical reaction caused by the immune system; requires compatible history AND evidence of immune mechanism | Yes | Can be (anaphylaxis) | Formal allergy assessment; consider AAI; refer if complex or severe |
| Sensitisation | Immune system has produced specific IgE to an allergen — test may be positive BUT patient may tolerate the substance without symptoms | Yes (immune memory, no reaction yet) | Not in itself | Positive test alone does NOT mandate avoidance. Correlate with history. Do not remove a tolerated food based on test alone. |
| Intolerance | Non-immune adverse reaction (e.g., lactose intolerance = enzyme deficiency, not immune). May be real and troublesome. | No | No | No allergy testing needed. Clinical diagnosis. No allergy label. |
| Sensitivity | A vague, non-specific term used by patients and sometimes clinicians. May mean “this upsets me,” “I had a positive test,” or “I don’t tolerate this well.” | Unclear (as used) | Unknown without clarification | Clarify: do they mean allergy, sensitisation, intolerance, or an unpleasant reaction? Don’t accept the term at face value. |
| Side effect | Predictable, dose-related, non-immune drug effect (e.g., GI upset with erythromycin, nausea with codeine) | No | Usually not | Do not label as allergy. Document accurately to prevent inappropriate avoidance. |
⚠️ The Tolerated Food Pearl
If a child is already eating a food without symptoms, a positive test alone is NOT a good reason to advise the family to stop that food. In some children, continued regular ingestion of a tolerated food may help maintain tolerance, whereas unnecessary withdrawal may risk loss of tolerance. UK paediatric allergy referral guidance states that children who are sensitised to, but tolerant of, a food may lose tolerance if that food is withdrawn. BSACI early feeding guidance also warns that unnecessary exclusion or delayed introduction of allergenic foods may increase later food allergy risk. Source: BSACI; NICE CKS.
Why Broad SPT ‘Battery Testing’ is Poor Practice
Allergy specialists prefer targeted testing based on a good clinical history. Large untargeted skin prick panels often create confusion rather than clarity. NICE and BSACI guidance support history-led assessment, with testing used to answer a specific clinical question about suspected IgE-mediated allergy. Source: NICE CKS; BSACI.
The history comes first. Testing should answer a specific clinical question, not go fishing for one. If the history is poor, the test result is much more likely to mislead.
A positive skin prick test often means sensitisation, not necessarily allergy. A patient may be test-positive and still tolerate the food clinically without reaction.
The more allergens tested, the more likely irrelevant positive results will be generated. This can lead to: incorrect food avoidance; unnecessary parental anxiety; over-labelling of patients as allergic; nutritional problems in children.
Removing a food that is actually being tolerated — based on a positive test alone — may lead to loss of tolerance. A food should not be removed just because a test is positive if the patient is eating it without symptoms. Source: BSACI.
SPT does not diagnose most delayed, non-IgE-mediated conditions. For many children with delayed GI symptoms or eczema flares, SPT will be negative and will not answer the clinical question.
Some food allergens are unstable and some extracts do not represent the exact allergen that caused the real-life reaction. Another reason results must always be interpreted in clinical context.
Sensitivity and Specificity of Skin Prick Tests
There is no single sensitivity or specificity for SPT — it varies with: the allergen tested, the extract quality, the patient’s age group, the wheal cut-off used, and the pre-test probability of disease.
Negative SPT: Generally Useful
SPT often has a good negative predictive value for IgE-mediated allergy. If the history suggests immediate allergy and the test is negative, this can make IgE-mediated allergy less likely.
Positive SPT: Weak Positive Predictive Value
A positive result does NOT reliably prove clinical allergy, especially when pre-test probability is low or testing has been broad and untargeted. Always interpret in light of clinical history.
Is SPT Operator-Dependent?
Yes. SPT is not simply “put the drop on and read the answer.” Results can be affected by:
- • Technique: depth and angle of the prick
- • Quality and storage of reagents and controls
- • Whether antihistamines were stopped adequately beforehand (3–7 days)
- • Presence of dermographism or active eczema (can cause false positives)
- • How the wheal is measured and recorded
Standardised allergy guidance emphasises trained staff, correct technique, and interpretation in clinical context. This is another reason casual or indiscriminate testing is poor practice. Source: Standardised allergy guidance; BSACI.
🎯 Core Take-Home Messages (Source: NICE CKS; BSACI)
General Allergy History Framework
- Presenting Complaint: What symptoms? When did they start? How long do they last?
- Trigger Identification: What were you doing/eating/exposed to just before symptoms? Any pattern (seasonal, occupational, food-related)?
- Symptom Characterisation: Skin (rash, itch, swelling)? Respiratory (wheeze, cough, SOB)? GI (nausea, vomiting, diarrhoea)? Cardiovascular (dizziness, collapse)?
- Timing: Immediate (within minutes–2 hours = likely IgE-mediated) or delayed (hours–days = likely non-IgE-mediated)? Reproducible?
- Severity: Ever needed emergency treatment? Hospital admission? Adrenaline?
- Previous Episodes: First time or recurrent? Getting worse over time?
- Atopic History: Asthma, eczema, hay fever? Family history of atopy?
- Current Management: What have you tried? What works? Any adrenaline auto-injectors?
- Impact: Effect on daily life, work, school, sleep, mental health?
- Red Flags: Airway compromise, cardiovascular instability, severe systemic symptoms?
Anaphylaxis History Framework
- Onset: How quickly did symptoms develop after exposure? (Typically minutes to 2 hours)
- ABC Involvement: Any difficulty breathing, wheeze, stridor, throat tightness? Dizziness, collapse, loss of consciousness?
- Skin/Mucosal: Urticaria, angioedema, flushing, itching? (Absent in 10–20% of cases — do not wait for skin signs if ABC compromised)
- GI Symptoms: Abdominal pain, vomiting, diarrhoea?
- Trigger: Food (especially nuts, shellfish, eggs, milk)? Insect sting? Medication? Exercise? Latex?
- Emergency Treatment: Was adrenaline given? How many doses? Ambulance called? Hospital admission?
- Biphasic Reaction: Did symptoms return hours later?
- Previous Episodes: Ever happened before? Getting more severe?
- Asthma: Do you have asthma? How well controlled? (Poorly controlled asthma increases anaphylaxis severity)
- Current AAI: Do you carry one? Have you been trained? In date?
💊 Prescribing Pearl: Adrenaline Auto-Injector
Always prescribe TWO devices. Dose: 300mcg (EpiPen 300mcg or Jext 300mcg) for adults and children >30kg; 150mcg (EpiPen Jr 150mcg or Jext 150mcg) for children 15–30kg. Demonstrate technique in clinic. Provide written emergency action plan. Review annually and after any use. Source: NICE CG134.
Urticaria History Framework
- Duration: Acute (<6 weeks) or chronic (>6 weeks)?
- Appearance: Raised, itchy wheals? Do individual lesions last <24 hours? (If >24 hours, consider urticarial vasculitis)
- Triggers: Food, medication, infection, physical triggers (pressure, cold, heat, exercise, water)?
- Angioedema: Any swelling of lips, tongue, eyes, hands, feet?
- Systemic Symptoms: Breathing difficulty, dizziness, GI symptoms? (Suggests anaphylaxis)
- Pattern: Constant or intermittent? Worse at certain times of day?
- Previous Treatment: Antihistamines tried? Effective dose? Duration?
- Impact: Sleep disturbance? Work/school absence? Quality of life?
- Associated Conditions: Thyroid disease, autoimmune conditions, chronic infections?
- Red Flags: Bruising, joint pain, fever (suggests vasculitis or systemic disease)
- Food trigger? Be cautious: unless there is a close temporal relationship to a specific food, IgE-mediated food allergy is usually NOT the cause of chronic urticaria. (Source: BSACI)
💊 Prescribing Pearl: Chronic Urticaria
First-line: Non-sedating antihistamine — cetirizine 10mg OD, loratadine 10mg OD, or fexofenadine 180mg OD. If inadequate after 2 weeks, increase dose cautiously (doubling to 20mg OD supported by NICE ESUOM31; up to 40mg daily off-label). Review after 2–4 weeks. Refer to dermatology if not controlled after 6 weeks. Source: NICE CKS Urticaria; NICE ESUOM31.
Food Allergy History Framework
- Suspected Food: Which food(s)? How certain are you? Single food or multiple?
- Reaction Type: Immediate (IgE-mediated: minutes–2 hours) or delayed (non-IgE: hours–days)?
- Symptoms: Skin (urticaria, angioedema, eczema flare)? GI? Respiratory? Cardiovascular (dizziness, collapse)?
- Reproducibility: Happens every time you eat this food? How many times?
- Amount: Does it happen with tiny amounts (trace) or only larger quantities?
- Preparation: Raw vs cooked? (Some allergens are heat-labile)
- Co-factors: Exercise, alcohol, NSAIDs, infection at the time?
- Age of Onset: Childhood or adult-onset?
- Atopic History: Eczema, asthma, hay fever?
- Current Avoidance: Completely avoiding the food? Accidental exposures?
- Nutritional Impact: Multiple food avoidances? Growth concerns in children? Dietitian input?
🔑 Key Distinction: IgE vs Non-IgE Food Allergy
IgE-mediated: Rapid onset (minutes–2 hours), skin/respiratory/GI/cardiovascular symptoms, can be life-threatening. Diagnosed with skin prick test or specific IgE blood test.
Non-IgE-mediated: Delayed onset (hours–days), mainly GI symptoms. Common in infants (cow’s milk protein allergy). SPT/IgE testing usually negative. Diagnosed clinically with elimination and reintroduction trial. (NICE CKS: cow’s milk non-IgE symptoms typically 2–72 hours after exposure)
Drug Allergy History Framework (Source: NICE drug allergy guidance)
NICE guidance emphasises accurate assessment and clear recording. A structured history separates true allergy from side effect, intolerance, non-allergic rash, or coincidental viral exanthem.
- Drug Name: Exact name — generic AND brand name?
- Dose: What dose was being taken?
- Route: Oral, IV, topical, IM?
- Frequency: How often was it being taken?
- Duration before reaction: How many doses or how many days of treatment before the reaction occurred?
- Indication: Why were you taking it?
- Timing of onset: Immediate (minutes–1 hour); accelerated (1–72 hours); delayed (>72 hours)?
- Exact clinical features: Skin (rash, urticaria, angioedema, blistering)? Respiratory? GI? Systemic (fever, joint pain, organ involvement)?
- Severity: Mild (rash only)? Moderate (systemic symptoms)? Severe (anaphylaxis, DRESS, SJS/TEN)?
- Was the drug stopped?
- Treatment Required: Antihistamines? Steroids? Adrenaline? Hospital admission?
- Re-exposure: Has it been taken again since? Same reaction?
- Related Drugs: Reactions to structurally similar drugs?
- Documentation: Is it recorded in GP notes, hospital letters, allergy alert bracelet?
🚨 Red Flag: Severe Drug Reactions
DRESS Syndrome: Fever, rash, lymphadenopathy, eosinophilia, organ involvement. Onset 2–8 weeks after drug start. High mortality. Common culprits: allopurinol, anticonvulsants, antibiotics. Immediate hospital admission.
SJS/TEN: Blistering, skin detachment, mucosal involvement. Life-threatening. Common culprits: allopurinol, anticonvulsants, antibiotics, NSAIDs. Immediate hospital admission.
💊 Documentation Pearl
Always document: drug name, dose, route, frequency, duration before reaction, exact reaction, timing, severity. Avoid vague entries like “penicillin allergy” without detail. Many reported ‘allergies’ are side effects (e.g., GI upset with erythromycin) or childhood reactions that may no longer apply. Consider allergy clinic referral for penicillin allergy de-labelling if history suggests low risk.
Allergic Rhinitis History Framework
- Symptoms: Sneezing, runny nose, blocked nose, itchy nose/eyes/throat? Post-nasal drip, cough?
- Pattern: Seasonal (hay fever: spring/summer) or perennial (year-round)?
- Triggers: Pollen (tree, grass, weed)? House dust mite? Pets? Mould?
- Severity: Mild (minimal impact), moderate (troublesome), or severe (very troublesome, major impact on QoL/sleep)?
- Duration: Intermittent (<4 days/week or <4 weeks) or persistent (>4 days/week and >4 weeks)?
- Associated Conditions: Asthma (allergic rhinitis and asthma are linked — poor rhinitis control worsens asthma), eczema, conjunctivitis, sinusitis, nasal polyps?
- Previous Treatment: Antihistamines? Nasal steroids? Eye drops? Effective? Compliance?
- Impact: Sleep disturbance? Work/school performance? Quality of life?
- Red Flags: Unilateral symptoms, blood-stained discharge, persistent blockage (consider structural causes, polyps, tumour)
💊 Prescribing Pearl: Allergic Rhinitis Treatment Ladder (Source: NICE CKS Allergic Rhinitis)
Step 1: Oral non-sedating antihistamine — cetirizine 10mg OD, loratadine 10mg OD, or fexofenadine 180mg OD — OR intranasal corticosteroid — fluticasone propionate 50mcg/spray, 2 sprays each nostril OD; or mometasone furoate 50mcg/spray, 2 sprays each nostril OD.
Step 2: Combine oral antihistamine + intranasal corticosteroid.
Step 3: Add intranasal antihistamine — azelastine 140mcg (1 spray) each nostril twice daily — or consider leukotriene receptor antagonist: montelukast 10mg OD.
Step 4: Refer to ENT/allergy for consideration of immunotherapy.
Immunodeficiency History Framework
- Infection Frequency: >8 new ear infections per year? >2 serious sinus infections per year? >2 pneumonias per year? >2 months on antibiotics with little effect?
- Infection Severity: Recurrent deep skin/organ abscesses? Persistent thrush (oral/oesophageal) after age 1? Need for IV antibiotics to clear infections?
- Unusual Organisms: Opportunistic infections (PCP, CMV, atypical mycobacteria)? Severe/recurrent viral infections?
- Failure to Thrive: Poor growth in children? Chronic diarrhoea? Weight loss?
- Family History: Early deaths from infection? Consanguinity? Known immunodeficiency?
- Autoimmunity: ITP, haemolytic anaemia, inflammatory bowel disease?
- Vaccine Reactions: Severe reaction to live vaccines (BCG, MMR)?
- HIV Risk: Sexual history, IVDU, blood transfusions, endemic area travel?
- Secondary Causes: Medications (steroids, immunosuppressants, chemotherapy)? Malignancy? Chronic disease?
🔬 Initial Investigations for Suspected Immunodeficiency
First-line: FBC (lymphocyte count), immunoglobulins (IgG, IgA, IgM), protein electrophoresis. Consider HIV test if risk factors.
Refer to immunology if: Low immunoglobulins, recurrent serious infections, opportunistic infections, or strong clinical suspicion despite normal basic tests.
When Allergy Testing IS Useful
- Suspected IgE-mediated food allergy: To confirm trigger and guide avoidance/reintroduction. Testing should be targeted to the specific food identified by history.
- Anaphylaxis: To identify trigger (food, venom, drug) and guide management.
- Allergic rhinitis: If diagnosis unclear or considering immunotherapy.
- Occupational allergy: To confirm sensitisation (e.g., latex, flour).
- Drug allergy: Specialist testing for penicillin allergy de-labelling.
- Venom allergy: After systemic reaction to bee/wasp sting (for immunotherapy consideration).
When Allergy Testing is NOT Useful
- Chronic urticaria: Allergy testing rarely identifies a cause (most cases are idiopathic). BSACI notes that unless there is a close temporal relationship to a food trigger, food allergy can usually be excluded without testing.
- Non-IgE food allergy: SPT and specific IgE tests are negative by definition. Diagnosis is clinical (elimination + reintroduction).
- Food intolerance: IgG food testing is not validated and should not be used.
- Eczema without clear food trigger: Routine food allergy testing not recommended.
- Broad untargeted allergy panels: No role for ‘allergy panels’ without a specific clinical question from the history. They increase false positives and may prompt unnecessary food avoidance.
⚠️ Key Principle: History is King
Allergy tests show sensitisation, not necessarily clinical allergy. A positive test without a convincing history does NOT mean the patient is allergic. Conversely, a negative test does not always exclude allergy (especially in non-IgE reactions). Always interpret tests in the context of the clinical history. (Source: NICE CKS; BSACI)
Skin Prick Testing (SPT): What, When, and Why Targeted Only
What it tests: IgE-mediated sensitisation to specific allergens.
How it works: Allergen extract placed on skin, pricked with lancet. Positive = wheal ≥3mm larger than negative control at 15 minutes.
Common allergens: Foods (milk, egg, peanut, tree nuts, fish, shellfish, wheat, soy), aeroallergens (grass/tree pollen, house dust mite, cat, dog, mould), venom (bee, wasp).
Advantages: Quick (15 minutes), cheap, high sensitivity, immediate results.
Disadvantages: Requires stopping antihistamines 3–7 days before. Risk of systemic reaction (rare). Not suitable if severe eczema or dermographism.
Where done: Specialist allergy clinics (not routinely in GP).
⚠️ Sensitivity, Specificity, and Operator Dependence
Good negative predictive value for IgE-mediated allergy. A negative test in a patient with a convincing history of immediate allergy can usefully reduce the likelihood of IgE sensitisation.
Does NOT reliably prove clinical allergy, especially with low pre-test probability or broad untargeted panels. Always interpret in clinical context.
🚨 Avoid: Broad Untargeted SPT Panels
Do not order broad ‘allergy batteries’ without a specific clinical question from the history. The more allergens tested, the more likely irrelevant positive results will appear, leading to: incorrect food avoidance; unnecessary parental anxiety; over-labelling; and nutritional harm in children. In some sensitised-but-tolerant children, withdrawing a food based on a test result alone may risk loss of tolerance. (Source: NICE CKS; BSACI)
Specific IgE Blood Testing
What it tests: IgE antibodies to specific allergens in blood.
Results: Reported as kU/L (or class 0–6). Positive = detectable specific IgE (usually >0.35 kU/L).
Advantages: No need to stop antihistamines. Safe. Can be done in GP. Useful if severe eczema or dermographism.
Disadvantages: More expensive than SPT. Takes days for results. Slightly lower sensitivity than SPT.
When to use in GP: Suspected IgE-mediated food allergy, anaphylaxis trigger identification, allergic rhinitis (if considering immunotherapy). Test specific foods only — not broad panels.
🚨 Avoid: IgG Food Testing
IgG food antibody tests are NOT validated and should NOT be used. IgG antibodies to food are a normal response to eating food, not a sign of allergy or intolerance. Commercial IgG food tests lead to unnecessary food avoidance and have no evidence base. Do not order them.
Patch Testing
What it tests: Delayed (Type IV) hypersensitivity reactions (contact dermatitis).
How it works: Allergens applied to skin under patches for 48 hours. Skin examined at 48 and 96 hours for eczematous reaction.
Common allergens: Metals (nickel, cobalt, chromate), fragrances, preservatives, rubber chemicals, topical medications, cosmetics.
Indications: Suspected allergic contact dermatitis (e.g., hand eczema, facial dermatitis, occupational dermatitis).
Where done: Specialist dermatology clinics (not in GP).
🔑 Patch Testing vs Skin Prick Testing
Patch testing: For delayed (Type IV) reactions. Tests contact dermatitis. Takes 4 days. Done by dermatology.
Skin prick testing: For immediate (Type I) IgE-mediated reactions. Tests food/aeroallergen allergy. Takes 15 minutes. Done by allergy clinic.
When to Refer to Immunology
- Recurrent serious infections: >2 pneumonias, >8 ear infections/year, >2 serious sinus infections/year, need for IV antibiotics.
- Opportunistic infections: PCP, CMV, atypical mycobacteria, severe/recurrent viral infections.
- Low immunoglobulins: IgG, IgA, or IgM below normal range.
- Failure to thrive: Poor growth, chronic diarrhoea, weight loss in context of recurrent infections.
- Family history of immunodeficiency
- Autoimmunity with recurrent infections: ITP, haemolytic anaemia, IBD.
- Severe reaction to live vaccines: BCG, MMR.
- Persistent lymphopenia: <1.0 × 10⁹/L in adults.
🔬 Initial Tests Before Immunology Referral
First-line: FBC (lymphocyte count), immunoglobulins (IgG, IgA, IgM), protein electrophoresis.
Consider: HIV test (if risk factors), vaccine antibody titres (tetanus, pneumococcal), complement (C3, C4) if recurrent bacterial infections.
| Feature | Urticaria | Angioedema | Anaphylaxis |
|---|---|---|---|
| Skin involvement | Raised, itchy wheals (hives) | Deep swelling of dermis/subcutis | Often urticaria + angioedema |
| Location | Anywhere on body | Lips, tongue, eyes, hands, feet, genitals | Widespread |
| Itch | Intense itch | Usually no itch (may be painful) | Variable |
| Duration | Individual wheals <24 hours | Swelling lasts 24–72 hours | Minutes to hours |
| Respiratory | No | If tongue/throat involved → airway risk | Wheeze, stridor, SOB, throat tightness |
| Cardiovascular | No | No | Hypotension, tachycardia, dizziness, collapse |
| Life-threatening? | No | Yes (if airway involved) | Yes |
| First-line treatment | Non-sedating antihistamine (cetirizine 10mg OD) | Antihistamines; IM adrenaline if airway threatened | IM adrenaline 500mcg, call 999 |
| Term | Immune-Mediated? | Life-threatening? | GP Action |
|---|---|---|---|
| Allergy | Yes (compatible history + immune mechanism) | Can be (anaphylaxis) | Formal assessment; consider AAI; avoid trigger; refer if complex |
| Sensitisation | Yes (IgE produced, but no clinical reaction yet) | Not in itself | Positive test ≠ allergy. Do NOT remove a tolerated food on test result alone |
| Intolerance | No (enzyme deficiency, pharmacological effect) | No | No allergy label. No allergy testing. Clinical management. |
| Drug side effect | No (predictable, dose-related) | Usually not | Do not label as allergy. Document accurately. |
💬 Patient Explanation Script
“An allergy involves your immune system and can be life-threatening. Even a tiny amount can trigger a reaction. An intolerance is unpleasant but not dangerous — it’s usually dose-dependent, so small amounts might be tolerated. If you’ve ever needed an ambulance after eating it, that’s an allergy. If you just feel bloated or uncomfortable, that’s likely intolerance.”
| Feature | IgE-mediated | Non-IgE-mediated |
|---|---|---|
| Mechanism | Specific IgE → mast cell degranulation → histamine | T-cell mediated (Type IV) |
| Onset | Rapid (minutes to 2 hours) | Delayed (hours to days; CMPA: 2–72 hours per NICE CKS) |
| Symptoms | Urticaria, angioedema, wheeze, vomiting, anaphylaxis | GI (vomiting, diarrhoea, reflux, constipation, blood in stool), eczema, feed refusal, faltering growth |
| Life-threatening? | Yes (anaphylaxis risk) | No |
| Testing | Skin prick test, specific IgE blood test (targeted) | No validated tests. Clinical diagnosis: elimination + reintroduction |
| Common foods | Peanut, tree nuts, shellfish, fish, egg, milk, wheat, soy | Cow’s milk, soy, wheat, egg (especially in infants) |
| Prognosis | Milk/egg often outgrown; nut/shellfish usually lifelong | Most outgrow by age 3–5 |
| Feature | Acute Urticaria | Chronic Urticaria |
|---|---|---|
| Duration | <6 weeks | >6 weeks |
| Cause | Often identifiable (infection, drug, food, insect sting) | Usually idiopathic (no cause in 80–90%). Not usually food allergy. |
| Food allergy? | Possible (if close temporal link to food) | Usually NOT. BSACI: IgE-mediated food allergy can be excluded unless close temporal relationship exists |
| Investigation | Usually none needed (self-limiting) | Limited: FBC, ESR/CRP, TFTs. Avoid broad allergy panels. |
| Treatment | Non-sedating antihistamine as required | Non-sedating antihistamine (up to 4× dose off-label); refer if uncontrolled after 6 weeks |
| Prognosis | Resolves within days–weeks | 50% resolve within 6 months; 20% persist >5 years |
🚨 10 Warning Signs of Primary Immunodeficiency
- ≥8 new ear infections within 1 year
- ≥2 serious sinus infections within 1 year
- ≥2 months on antibiotics with little effect
- ≥2 pneumonias within 1 year
- Failure of infant to gain weight or grow normally
- Recurrent deep skin or organ abscesses
- Persistent thrush in mouth or fungal infection on skin after age 1
- Need for IV antibiotics to clear infections
- ≥2 deep-seated infections (meningitis, osteomyelitis, sepsis)
- Family history of primary immunodeficiency
🔬 Initial Investigations
First-line: FBC (lymphocyte count), immunoglobulins (IgG, IgA, IgM), protein electrophoresis.
Refer to immunology if: Low immunoglobulins, persistent lymphopenia (<1.0 × 10⁹/L), recurrent serious infections despite normal basic tests, opportunistic infections, family history.
Key History Questions
- Symptoms: Sneezing, runny nose, blocked nose, itchy nose/eyes/throat, post-nasal drip
- Pattern: Seasonal (spring/summer = pollen) or perennial (year-round = dust mite/pets)?
- Triggers: Pollen, house dust mite, pets, mould?
- Severity: Mild (minimal impact), moderate (troublesome), severe (major impact on QoL/sleep)?
- Duration: Intermittent (<4 days/week or <4 weeks) or persistent (>4 days/week and >4 weeks)?
- Associated conditions: Asthma (allergic rhinitis often worsens asthma control — ask about both), eczema, conjunctivitis, sinusitis?
- Previous treatment: Antihistamines? Nasal steroids? Compliance?
Examination Findings
- Nasal mucosa: Pale, swollen, boggy (allergic) vs red, inflamed (infective)
- Nasal discharge: Clear, watery (allergic) vs purulent (infective/sinusitis)
- Nasal polyps: Smooth, pale, grape-like masses (refer to ENT)
- Eyes: Conjunctival injection, chemosis, periorbital oedema
- Chest: Wheeze (check for asthma)
- Skin: Eczema, urticaria
Investigations
Usually none needed in primary care. Diagnosis is clinical.
- Allergy testing: Consider if diagnosis unclear or considering immunotherapy. Specific IgE or skin prick test at specialist clinic.
- Nasal endoscopy: If unilateral symptoms, polyps suspected, or red flags. Refer to ENT.
💊 Treatment Ladder (Source: NICE CKS Allergic Rhinitis)
- Oral non-sedating antihistamine: cetirizine 10mg OD, loratadine 10mg OD, or fexofenadine 180mg OD
- OR intranasal corticosteroid (preferred for nasal blockage): fluticasone propionate 50mcg/spray, 2 sprays each nostril OD; or mometasone furoate 50mcg/spray, 2 sprays each nostril OD
- Combine oral antihistamine + intranasal corticosteroid
- Add topical antihistamine eye drops if eye symptoms
- Intranasal antihistamine: azelastine 140mcg (1 spray) each nostril twice daily
- OR leukotriene receptor antagonist: montelukast 10mg OD
- Refer to ENT/allergy for allergen immunotherapy consideration
🌿 Allergen Avoidance Advice
- Pollen: Keep windows closed, shower after being outdoors, wear wraparound sunglasses, avoid grassy areas when pollen count high
- House dust mite: Use allergen-proof mattress/pillow covers, wash bedding weekly at 60°C, reduce soft furnishings, vacuum with HEPA filter
- Pets: Keep pets out of bedroom, wash pets weekly, use air purifiers with HEPA filter
When to Refer to ENT/Allergy
- Symptoms not controlled after 3-step treatment ladder
- Unilateral symptoms (consider structural cause, polyp, tumour)
- Blood-stained nasal discharge
- Nasal polyps on examination
- Suspected occupational rhinitis
- Considering immunotherapy
- Diagnostic uncertainty
Key History Questions
- Duration: Acute (<6 weeks) or chronic (>6 weeks)?
- Appearance: Raised, itchy wheals? Do individual lesions last <24 hours? (If >24 hours, consider urticarial vasculitis)
- Triggers: Food, medication, infection, physical triggers (pressure, cold, heat, exercise, water)?
- Food trigger? Only relevant if there is a close temporal relationship to a specific food. In chronic urticaria, IgE-mediated food allergy can usually be excluded if this is absent. (Source: BSACI)
- Angioedema: Any swelling of lips, tongue, eyes, hands, feet?
- Systemic symptoms: Breathing difficulty, dizziness? (Suggests anaphylaxis)
- Pattern: Constant or intermittent? Worse at certain times?
- Previous treatment: Antihistamines tried? Effective dose?
- Impact: Sleep disturbance? Work/school absence?
Examination Findings
- Wheals: Raised, erythematous, blanching, variable size/shape, central pallor
- Distribution: Anywhere on body
- Dermographism: Stroke skin firmly — wheal within 5 minutes = dermographism
- Angioedema: Non-pitting swelling of lips, tongue, eyes, hands, feet
- Red flags: Bruising (vasculitis), fixed lesions >24 hours, fever, joint pain
Investigations
Acute urticaria: Usually none needed.
Chronic urticaria (if not responding to treatment): FBC, ESR/CRP, TFTs.
Avoid broad allergy panels in chronic urticaria — they rarely identify a cause and often create false positives and unnecessary food avoidance. (Source: NICE CKS Urticaria; BSACI)
💊 Treatment Protocol (Source: NICE CKS Urticaria; NICE ESUOM31; BSACI)
- cetirizine 10mg OD, loratadine 10mg OD, or fexofenadine 180mg OD
- Trial for 2 weeks before reassessing
- Initial up-titration: cetirizine 20mg OD (or loratadine 20mg OD, fexofenadine 360mg OD) — supported by NICE ESUOM31
- Up to 4× licensed dose (e.g., cetirizine 10mg QDS, fexofenadine 180mg QDS) — off-label but BSACI/EAACI-endorsed; document rationale; avoid in pregnancy
- Short course oral prednisolone: prednisolone 0.5mg/kg/day (approximately 30–40mg OD for most adults) for 3–5 days — Source: NICE CKS; BSACI. Avoid long-term or repeated courses.
- Specialist may consider omalizumab or ciclosporin (secondary care only)
⚠️ Key Message
Most chronic urticaria is idiopathic. Extensive investigation rarely changes management. Do not label as food allergy without clear evidence. Reassure patients: 50% resolve within 6 months, though 20% persist >5 years.
When to Refer to Dermatology
- Not controlled after 6 weeks of optimised antihistamine treatment
- Suspected urticarial vasculitis: Individual lesions >24 hours, bruising, pain, systemic symptoms
- Angioedema without urticaria: Consider hereditary angioedema or ACE inhibitor cause
- Diagnostic uncertainty
- Severe impact on quality of life despite treatment
Key History Questions
- Location: Lips, tongue, eyes, hands, feet, genitals? Throat/airway involvement?
- Onset: Rapid (minutes–hours) or gradual?
- Duration: How long does swelling last? (Typically 24–72 hours)
- Associated urticaria: Hives present? (If yes, likely histamine-mediated)
- Triggers: Food, medication, insect sting?
- Medications: ACE inhibitors (can cause angioedema even after years of use), NSAIDs?
- Family history: Recurrent angioedema? (Consider hereditary angioedema)
- Airway symptoms: Difficulty breathing, stridor, voice change, difficulty swallowing?
Examination Findings
- Swelling: Non-pitting, non-erythematous, deep dermal/subcutaneous
- Colour: Normal skin colour (not red like cellulitis)
- Urticaria: Present or absent?
- Airway assessment: Stridor, drooling, voice change, respiratory distress? (EMERGENCY)
- Tongue/throat: Swelling, uvula swelling, difficulty swallowing?
Investigations
Angioedema without urticaria (recurrent):
- C4 level: Low in hereditary angioedema (HAE) and acquired angioedema — check first
- C1 esterase inhibitor level and function: If C4 low
- Medication review: ACE inhibitors — stop and switch if suspected
🚨 EMERGENCY: Airway-Threatening Angioedema (Source: RCUK 2021)
- Call 999 immediately
- IM adrenaline 500mcg (0.5mL of 1:1000) into anterolateral thigh — repeat after 5 minutes if no improvement
- High-flow oxygen; sit patient upright
- Do NOT discharge — admit for observation
- Note: IV chlorphenamine and IV hydrocortisone are no longer routinely recommended (RCUK 2021). Adrenaline IM is the priority.
💊 Non-Emergency Management
- Non-sedating antihistamine: cetirizine 10mg OD, loratadine 10mg OD, or fexofenadine 180mg OD
- Short course oral prednisolone: prednisolone 0.5mg/kg/day (approximately 30–40mg OD) for 3–5 days if severe
- Stop ACE inhibitor immediately (e.g., ramipril, lisinopril, perindopril, enalapril)
- Switch to ARB: losartan, candesartan, or irbesartan at doses per NICE hypertension guidelines (very low cross-reactivity risk)
- Antihistamines and steroids NOT effective (bradykinin-mediated, not histamine-mediated)
- Can occur even after years of stable use
- Refer urgently to immunology. Treatments (C1-INH concentrate, icatibant, FFP) are secondary care only.
- Antihistamines, steroids, and adrenaline are NOT effective for HAE
When to Refer
- Recurrent angioedema without urticaria: Refer to immunology (consider HAE); check C4 before referral
- Low C4 level: Urgent immunology referral
- Family history of recurrent angioedema
- Angioedema not responding to antihistamines/steroids
- Any airway involvement: Emergency admission
Key History Questions
- Suspected food: Which food(s)? Single or multiple?
- Reaction type: Immediate (within minutes–2 hours = IgE-mediated) or delayed (hours–days = non-IgE-mediated)?
- Symptoms: Skin (urticaria, angioedema, eczema flare)? GI (vomiting, diarrhoea, reflux, constipation, blood/mucus in stool)? Respiratory? Cardiovascular (dizziness, collapse)?
- Reproducibility: Happens every time? How many times?
- Amount: Trace amounts or larger quantities?
- Preparation: Raw vs cooked? (Some allergens are heat-labile)
- Age of onset: Childhood or adult-onset?
- Atopic history: Eczema, asthma, hay fever?
- Current avoidance: Completely avoiding? Accidental exposures?
- Nutritional impact: Multiple food avoidances? Growth concerns in children? Dietitian involved?
Examination Findings
- Skin: Eczema (especially in infants with non-IgE CMPA), urticaria, angioedema
- Growth: Weight/height centiles (multiple food avoidances can affect growth)
- Chest: Wheeze (asthma increases anaphylaxis risk)
- Abdomen: Distension, tenderness (in non-IgE GI allergy)
Investigations
IgE-mediated food allergy (immediate reactions):
- Specific IgE blood test: Can be done in GP. Test for suspected specific food(s) only — not broad panels. A positive result = sensitisation, not necessarily clinical allergy. Interpret with history.
- Skin prick test: Done in specialist allergy clinic. More sensitive than specific IgE.
Non-IgE-mediated food allergy (delayed reactions):
- No validated tests. Diagnosis is clinical: elimination (2–4 weeks) then reintroduction. The “improve off milk, worsen on reintroduction” pattern is often more useful than any test.
- Avoid IgG food testing (not validated, leads to unnecessary avoidance).
💊 Management Principles
- Provide written advice on food labelling, cross-contamination, eating out
- Refer to dietitian if multiple food avoidances or growth concerns
- Prescribe adrenaline auto-injector if anaphylaxis risk: 300mcg (EpiPen 300mcg or Jext 300mcg) for adults and children >30kg; 150mcg (EpiPen Jr or Jext 150mcg) for children 15–30kg — always TWO devices (Source: NICE CG134)
- Provide written emergency action plan; demonstrate auto-injector technique in clinic
- cetirizine 10mg OD or loratadine 10mg OD for mild urticaria/itch until resolved
- NOT a substitute for adrenaline in anaphylaxis
- If a child is already eating a food without symptoms, a positive test alone is NOT a reason to remove that food. Regular ingestion of a tolerated food may help maintain tolerance. Unnecessary withdrawal may risk loss of tolerance. (Source: BSACI)
👤 Cow’s Milk Protein Allergy (CMPA) in Infants (Source: NICE CKS CMPA)
CMPA is one of the most important food allergy topics in UK primary care. Think of it when symptoms fit either an immediate or delayed pattern.
Immediate Clues (IgE-mediated)
- Urticaria or angioedema
- Vomiting soon after milk exposure
- Wheeze or cough
- Immediate skin reactions
Delayed Clues (non-IgE-mediated)
- Reflux-type symptoms
- Vomiting, diarrhoea, constipation
- Blood or mucus in stool
- Eczema (especially infantile)
- Feed refusal, irritability
- Faltering growth
💊 Formula Pathway (Source: NICE CKS CMPA)
- Extensively hydrolysed formula (eHF) — suitable for most infants with CMPA
- Amino acid formula (AAF) — indicated if: severe multisystem disease; faltering growth; very significant symptoms; eHF failed; or eosinophilic oesophagitis suspected
- Goat’s milk and sheep’s milk are NOT suitable substitutes — high cross-reactivity with cow’s milk protein
- Soya formula is generally avoided under 6 months (phytoestrogen concerns and cross-reactivity risk)
- Partially hydrolysed formula (pHF) is NOT suitable for CMPA treatment (insufficient hydrolysis)
🍼 Breastfed Infants with CMPA
If the infant is breastfed and symptomatic: The mother should trial a strict cow’s milk-free diet for 2–4 weeks with attention to nutritional adequacy (particularly calcium and vitamin D intake).
If symptoms improve: Continue maternal exclusion diet and review. Confirm by observing whether symptoms return when cow’s milk is reintroduced into mother’s diet.
Always refer to a paediatric dietitian if maternal exclusion diet is required, to ensure nutritional adequacy.
🔬 Diagnosing CMPA: The Elimination-Reintroduction Approach
For immediate (IgE) CMPA: Clinical history is key; allergy testing (specific IgE or SPT) may be used through the appropriate specialist pathway if the story is convincing.
For delayed (non-IgE) CMPA: Remove cow’s milk protein for 2–4 weeks. If symptoms improve, reintroduce to confirm. The “improve off milk, worsen on reintroduction” pattern is the key diagnostic criterion. Allergy tests will typically be negative.
Prognosis: Most infants outgrow CMPA by age 3–5 years. Planned reintroduction challenges should be undertaken under dietitian/allergy specialist guidance.
When to Refer to Allergy Clinic
- History of anaphylaxis to food
- Severe IgE-mediated food allergy (e.g., peanut, tree nut, shellfish)
- Multiple food allergies
- Diagnostic uncertainty (unclear if IgE or non-IgE, or if true allergy)
- Consideration of supervised food challenge
- Growth concerns due to multiple food avoidances (also refer to dietitian)
- Poorly controlled asthma + food allergy
- CMPA: Severe multisystem disease, faltering growth, failure of eHF, or uncertainty about diagnosis
Key History Questions (Source: NICE drug allergy guidance)
NICE guidance emphasises accurate assessment and clear recording. A structured history separates true allergy from side effect, intolerance, non-allergic rash, or coincidental viral exanthem.
- Drug name: Exact name — generic AND brand name?
- Dose: What dose was being taken?
- Route: Oral, IV, topical, IM?
- Frequency: How often was it being taken?
- Duration before reaction: How many doses or days of treatment before the reaction?
- Indication: Why were you taking it?
- Timing of onset: Immediate (minutes–1 hour); accelerated (1–72 hours); delayed (>72 hours)?
- Exact clinical features: Skin (rash type, urticaria, angioedema, blistering)? Respiratory? GI? Systemic (fever, joint pain, organ involvement)?
- Severity: Mild (rash only)? Moderate (systemic symptoms)? Severe (anaphylaxis, DRESS, SJS/TEN)?
- Was the drug stopped?
- Treatment required: Antihistamines? Steroids? Adrenaline? Hospital admission?
- Re-exposure: Ever taken again? Same reaction?
- Related drugs: Reactions to similar drugs?
💊 How to Document Drug Allergies (Source: NICE)
Always document: (1) Drug name, (2) Dose, (3) Route, (4) Frequency, (5) Duration before reaction, (6) Exact reaction, (7) Timing, (8) Severity. The allergy status should be clearly visible and kept separate from other adverse drug reactions.
⚠️ Common Pitfalls
- Side effects labelled as allergies: GI upset (erythromycin), headache (nitrofurantoin), nausea (codeine) are NOT allergies.
- Childhood reactions: Many childhood ‘penicillin allergies’ are viral rashes or no longer relevant. Consider de-labelling.
- Vague entries: “Allergic to antibiotics” without detail leads to unnecessary broad-spectrum use and antimicrobial resistance.
💊 Management Principles
- Mild (rash/urticaria only): Non-sedating antihistamine — cetirizine 10mg OD or loratadine 10mg OD until rash resolves
- Moderate (systemic symptoms): Non-sedating antihistamine + short course oral prednisolone — prednisolone 30–40mg OD for 3–5 days (verify dose per patient weight against BNF)
- Severe (anaphylaxis): IM adrenaline 500mcg (0.5mL of 1:1000), call 999. Source: RCUK 2021.
- DRESS/SJS/TEN: Stop drug immediately; immediate hospital admission
- Penicillin allergy: avoid all penicillins. Low cross-reactivity with cephalosporins (<2%) but avoid if severe reaction.
- NSAID allergy: avoid all NSAIDs if true allergy.
- Consider allergy alert bracelet for severe reactions.
🚨 Red Flag: Severe Drug Reactions
DRESS Syndrome: Fever, rash, lymphadenopathy, eosinophilia, organ involvement (liver, kidney). Onset 2–8 weeks after drug start. Mortality up to 10%. Common culprits: allopurinol, anticonvulsants (carbamazepine, phenytoin, lamotrigine), sulfonamides. Immediate hospital admission.
SJS/TEN: Blistering, skin detachment, mucosal involvement. Life-threatening (TEN mortality ~30%). Common culprits: allopurinol, anticonvulsants, antibiotics, NSAIDs (oxicam group). Immediate hospital admission; burns unit for severe cases.
When to Refer to Allergy Clinic
- Penicillin allergy de-labelling: If history suggests low risk (childhood reaction, vague history, mild rash), refer for testing to restore antibiotic options and support antimicrobial stewardship.
- Multiple drug allergies limiting treatment options.
- Need for drug challenge to confirm or exclude allergy.
- Severe drug reactions (DRESS, SJS/TEN) for future guidance after acute management.
- Diagnostic uncertainty between allergy and side effect.
💊 Who Needs an Adrenaline Auto-Injector? (Source: NICE CG134)
- Previous anaphylaxis (any cause)
- IgE-mediated food allergy with any of: asthma, previous systemic reaction, allergy to peanut/tree nut/shellfish/fish
- Venom allergy with previous systemic reaction
- Idiopathic anaphylaxis
- Exercise-induced anaphylaxis
💊 Prescribing Details (Source: NICE CG134)
- Adults and children >30kg: 300mcg — EpiPen 300mcg or Jext 300mcg
- Children 15–30kg: 150mcg — EpiPen Jr 150mcg or Jext 150mcg
- Children <15kg: Seek specialist advice (paediatric allergy)
- Demonstrate technique using trainer device in clinic
- Provide written emergency action plan
- Advise to carry both devices at all times
- Inform family/school/workplace
- Check expiry dates at each review
- Annual review (expiry, technique, emergency plan)
- Review after any use (replace device, assess trigger, consider allergy referral)
🚨 When to Use the Adrenaline Auto-Injector
Use immediately if any of the following after allergen exposure:
- Airway: Throat tightness, difficulty swallowing, hoarse voice, stridor
- Breathing: Wheeze, shortness of breath, persistent cough
- Circulation: Dizziness, collapse, loss of consciousness, pale/clammy
- OR sudden onset skin changes + any of the above
After using: Call 999 immediately. Lie flat with legs raised (or semi-recumbent if breathing difficulty). Repeat after 5 minutes if no improvement. Go to hospital even if symptoms improve.
🚨 Anaphylaxis Criteria (Resuscitation Council UK 2021)
Anaphylaxis is likely when ALL of the following are present:
- Airway: Throat/tongue swelling, hoarse voice, stridor
- Breathing: Wheeze, shortness of breath, respiratory arrest
- Circulation: Hypotension, tachycardia, dizziness, collapse, cardiac arrest
- Urticaria, angioedema, flushing, itching
- Note: Skin changes can be subtle or absent in 10–20% of cases — do NOT wait for skin signs if ABC problems are present
Common Triggers (Source: NICE CG134)
Foods (peanut, tree nuts, shellfish, fish, egg, milk), insect venom (bee, wasp), drugs (antibiotics, NSAIDs, anaesthetic agents), latex. Some drug reactions may occur via non-allergic mechanisms (e.g., direct mast cell activation). Foods, insect venom, drugs, and latex are the most common triggers of IgE-mediated anaphylaxis in UK primary care.
🚨 EMERGENCY MANAGEMENT (Resuscitation Council UK 2021 Algorithm)
- Call for help / Call 999 — state “anaphylaxis”
- Remove trigger if possible — stop drug infusion; scrape (do not squeeze) bee sting
- IM Adrenaline 1:1000 into anterolateral thigh — repeat after 5 minutes if no improvement:
- Adults and children >12 years: 500mcg (0.5mL)
- Children 6–12 years: 300mcg (0.3mL)
- Children <6 years: 150mcg (0.15mL)
- Position: Lie flat with legs raised if hypotensive; semi-recumbent if breathing difficulty; recovery position if unconscious. Never stand the patient up suddenly (risk of cardiac arrest).
- High-flow oxygen (if available)
- IV access + fluid bolus if hypotensive: 500–1000mL 0.9% sodium chloride (adult)
- Monitor continuously: Pulse, BP, respiratory rate, SpO2, conscious level
- Transfer to hospital: All patients need minimum 6 hours observation (biphasic reaction risk)
🚨 RCUK 2021 UPDATE: IV chlorphenamine and IV hydrocortisone are NO LONGER routinely recommended. Adrenaline IM is the ONLY life-saving first-line treatment. After stabilisation, a non-sedating oral antihistamine (e.g., cetirizine 10mg) may help persisting skin symptoms. Steroids may be considered only for refractory anaphylaxis — they must NOT replace or delay adrenaline.
💊 Adrenaline Auto-Injector Prescribing Post-Anaphylaxis (Source: NICE CG134)
All patients with anaphylaxis should be prescribed TWO adrenaline auto-injectors before or at discharge.
- Adults and children >30kg: 300mcg (EpiPen 300mcg or Jext 300mcg)
- Children 15–30kg: 150mcg (EpiPen Jr 150mcg or Jext 150mcg)
- Demonstrate technique using trainer device
- Provide written emergency action plan
- Advise to carry both devices at all times
- Inform family/school/workplace
Post-Anaphylaxis Follow-up
- GP review within 1 week: Check two auto-injectors issued, understands use, has emergency action plan.
- Serum tryptase: If taken during acute episode (ideally 1–2 hours after symptom onset), elevated tryptase supports diagnosis.
- Trigger identification: Detailed history. Consider specific IgE or refer to allergy clinic.
- Asthma review: Ensure well-controlled (poorly controlled asthma increases anaphylaxis severity).
- Refer to allergy clinic: All patients with anaphylaxis should be referred for specialist assessment, trigger identification, and long-term management plan.
- Medical alert bracelet: Consider for recurrent anaphylaxis or unavoidable triggers (e.g., venom allergy).
When to Refer to Allergy Clinic
- All patients with anaphylaxis should be referred for specialist assessment
- Trigger identification: Allergy testing (skin prick, specific IgE)
- Venom allergy: Consider immunotherapy (desensitisation)
- Idiopathic anaphylaxis: No identifiable trigger despite investigation
- Recurrent anaphylaxis
- Poorly controlled asthma + anaphylaxis risk
🚨 Recognition
- Tongue swelling
- Throat tightness
- Difficulty swallowing (drooling)
- Voice change (muffled, hoarse)
- Stridor (high-pitched inspiratory noise)
- Respiratory distress
🚨 EMERGENCY MANAGEMENT (Source: RCUK 2021)
- Call 999 immediately
- IM adrenaline 500mcg (0.5mL of 1:1000) — repeat after 5 minutes if no improvement
- High-flow oxygen; sit patient upright
- IV access
- Do NOT discharge — admit for observation and airway monitoring
⚠️ Special Case: ACE Inhibitor-Induced Angioedema
ACE inhibitors (e.g., ramipril, lisinopril, perindopril, enalapril) can cause angioedema even after years of use. It is bradykinin-mediated (NOT histamine-mediated), so antihistamines and steroids have limited or no effect. Adrenaline may help if airway is threatened.
Management: Stop ACE inhibitor immediately. Switch to an ARB (e.g., losartan, candesartan, or irbesartan at doses per NICE hypertension guidelines) or an alternative antihypertensive class. If airway involvement: treat as above and admit.
🚨 DRESS Syndrome (Drug Reaction with Eosinophilia and Systemic Symptoms)
Recognition:
- Onset: 2–8 weeks after starting drug
- Fever (>38°C), malaise
- Widespread maculopapular rash (may become confluent)
- Facial oedema; lymphadenopathy
- Eosinophilia on blood test
- Organ involvement: Liver (hepatitis), kidney (nephritis), lung, heart
Common culprits: Allopurinol, anticonvulsants (carbamazepine, phenytoin, lamotrigine), antibiotics (sulfonamides, vancomycin, minocycline), dapsone
- Stop suspected drug immediately
- Immediate hospital admission (mortality up to 10%)
🚨 SJS/TEN (Stevens-Johnson Syndrome / Toxic Epidermal Necrolysis)
- Onset: 1–3 weeks after starting drug
- Prodrome: Fever, malaise, sore throat
- Mucosal involvement: Painful mouth ulcers, conjunctivitis, genital ulcers
- Skin: Erythematous macules → blistering → skin detachment
- Nikolsky sign positive: Gentle pressure causes skin to slough off
Common culprits: Allopurinol, anticonvulsants, sulfonamides, NSAIDs (especially oxicam group)
Stop drug immediately. Immediate hospital admission. SJS mortality ~5%; TEN mortality ~30%. Burns unit or ICU for severe cases.
🚨 High-Risk Groups
- Chemotherapy patients (especially if neutropenic)
- Immunosuppressant therapy (steroids, biologics, DMARDs)
- HIV/AIDS (especially if CD4 <200)
- Asplenia (post-splenectomy, sickle cell disease)
- Primary immunodeficiency
- Organ transplant recipients
🚨 EMERGENCY: Neutropenic Sepsis
Fever (>38°C) in a patient on chemotherapy = neutropenic sepsis until proven otherwise. Do NOT wait for blood results.
- Call 999 or arrange immediate hospital transfer
- Bloods if time permits: FBC, U&Es, LFTs, CRP, blood cultures
- IV broad-spectrum antibiotics within 1 hour (hospital administration — e.g., piperacillin-tazobactam per local protocol)
🚨 When to Consider HIV Testing
- Recurrent infections: Pneumonia, sinusitis, oral/oesophageal candidiasis, herpes zoster (<50 years), severe seborrhoeic dermatitis
- Opportunistic infections: PCP pneumonia, CMV, toxoplasmosis, atypical mycobacteria
- Unexplained weight loss, fever, night sweats, diarrhoea
- Lymphadenopathy (persistent, generalised)
- Oral hairy leukoplakia (white patches on lateral tongue)
- Kaposi’s sarcoma (purple/red skin lesions)
- Persistent lymphopenia (<1.0 × 10⁹/L)
- Sexual health risk factors: MSM, IVDU, partner from high-prevalence country, multiple sexual partners
🔬 HIV Testing
Offer HIV test to all patients with indicator conditions. Use 4th generation HIV test (detects HIV antibodies and p24 antigen). Window period: 45 days.
If positive: Same-day urgent referral to HIV clinic. Modern antiretroviral therapy is highly effective — early diagnosis saves lives and prevents onward transmission.
Vaccine questions in allergic patients come up frequently in real GP work — particularly uncertainty about previous reactions and what is or is not a true contraindication. Most concerns relate to egg allergy, previous vaccine reactions, or allergy to vaccine components. The UKHSA Green Book (Immunisation Against Infectious Disease) is the authoritative UK reference and should be consulted for any vaccine-specific contraindication query.
Key Principles (Source: UKHSA Green Book)
- True contraindications to vaccines are rare. The benefits of vaccination almost always outweigh the risks. Many patients are incorrectly labelled as unable to receive vaccines.
- Allergy to a vaccine component is a contraindication to that specific vaccine, not necessarily to other vaccines. Identify the specific component causing concern.
- Previous anaphylaxis to a vaccine (or a component of it) is a contraindication to that specific vaccine — specialist advice required before any further dose.
- Minor illnesses (mild fever, common cold) are NOT contraindications to vaccination. Vaccination should proceed.
- Atopic conditions (eczema, asthma, hay fever, food allergy) are NOT contraindications to vaccination. Most allergic patients can be vaccinated in the normal schedule.
- Family history of allergy is NOT a contraindication to vaccination.
⚠️ What IS and ISN’T a Contraindication
| NOT a contraindication | IS a contraindication |
|---|---|
| Mild febrile illness | Anaphylaxis to previous dose of same vaccine |
| Eczema, asthma, hay fever | Anaphylaxis to a specific vaccine component |
| Food allergy (unless egg allergy — see below) | Live vaccines in confirmed immunodeficiency (see below) |
| Family history of allergy | Live vaccines in pregnancy (generally) |
| Premature birth (delay if unwell) | Live vaccines within 4 weeks of other live vaccines |
Egg Allergy and Influenza Vaccine (Source: UKHSA Green Book, Chapter 19)
Influenza vaccines can be grown in eggs and may contain small amounts of egg protein (ovalbumin). The risk to egg-allergic patients has been extensively studied and guidance has evolved significantly.
- Mild egg allergy (urticaria only): Can receive standard influenza vaccine in primary care. No special precautions beyond usual post-vaccination observation (15 minutes).
- Egg allergy with previous anaphylaxis: Should receive influenza vaccine in a setting where anaphylaxis can be managed (e.g., hospital immunology/allergy clinic, or under specialist advice). Consider egg-free influenza vaccine (e.g., cell-based or recombinant vaccines where available).
- MMR vaccine: MMR is NOT contraindicated in egg-allergic patients (including those with severe egg allergy or previous anaphylaxis to egg). MMR is grown on chick embryo fibroblast cultures, not egg albumen. (Source: UKHSA Green Book, Chapter 21)
⚠️ MMR and Egg Allergy: A Common GP Pitfall
MMR is safe to give to children with egg allergy, including those who have had anaphylaxis to egg. This is a well-established and frequently misunderstood point. The concern arose because MMR contains minute traces of chick embryo antigens — but these are distinct from egg white (ovalbumin) and do not pose a risk. Withholding MMR from egg-allergic children is incorrect practice. (Source: UKHSA Green Book, Chapter 21; BSACI)
Assessing a Previous Vaccine Reaction (Source: UKHSA Green Book)
Not all reactions to vaccines are allergic. A careful history determines whether the reaction was a true hypersensitivity reaction or a common, expected, non-allergic response.
| Reaction Type | Examples | Contraindication to repeat? |
|---|---|---|
| Expected local/systemic reaction | Soreness at injection site, mild fever, irritability, mild rash | No |
| Extensive limb swelling | Large local reaction extending from injection site | Not usually — seek advice |
| Vasovagal episode | Fainting/near-fainting after injection (no anaphylaxis features) | No — lie patient flat for future doses |
| Anaphylaxis | ABC involvement, rapid onset, systemic allergic features | Yes — same vaccine/component. Specialist referral required before repeat. |
🔭 History Questions for Previous Vaccine Reaction
- Which vaccine? (Name and dose number)
- Timing: How quickly after the vaccine did symptoms start? (Anaphylaxis: within minutes–1 hour; immediate = more likely allergic)
- Symptoms: Exact features — urticaria, angioedema, wheeze, hypotension, collapse?
- Severity: Was adrenaline given? Hospital admission?
- Recovery: How long did it take? Full recovery?
- Subsequent vaccines: Has the same or similar vaccine been given since without reaction?
🚨 Live Vaccines in Immunocompromised Patients (Source: UKHSA Green Book)
Live attenuated vaccines can cause serious infection in immunocompromised patients. They are generally contraindicated in:
- Primary immunodeficiency (e.g., severe combined immunodeficiency, agammaglobulinaemia)
- HIV infection with low CD4 count (generally <200 cells/mm³)
- High-dose immunosuppressive therapy: Prednisolone ≥40mg/day for ≥1 week (adults), or equivalent; or high-dose biologics affecting cell-mediated immunity
- Chemotherapy or radiotherapy
- Organ or bone marrow transplant recipients
- Pregnancy (most live vaccines)
| Live Vaccines to be Cautious About | Notes |
|---|---|
| MMR | Contraindicated in significant immunodeficiency; may be given in HIV with CD4 >200 — specialist decision |
| Varicella (chickenpox) | Contraindicated in immunodeficiency; non-immune close contacts of immunodeficient patients may be vaccinated |
| BCG | Contraindicated in immunodeficiency; can cause disseminated BCG infection |
| Yellow fever, Typhoid (oral), Rotavirus | Generally contraindicated in immunodeficiency — seek specialist advice |
✅ Inactivated Vaccines in Immunocompromised Patients
Inactivated (killed) vaccines and toxoids are generally safe in immunocompromised patients, though efficacy may be reduced. Immunocompromised patients often need additional or booster doses. Always check the Green Book for the specific vaccine and condition. Examples: influenza (injectable), pneumococcal, hepatitis B, HPV, meningococcal, COVID-19.
⚠️ Steroids and Vaccination: Practical Guidance
Low-dose or short-course steroids (e.g., prednisolone <20mg/day for <14 days): Live vaccines can usually be given. (Source: UKHSA Green Book)
High-dose or long-term steroids: Live vaccines are generally contraindicated during treatment. Defer live vaccines until ≥3 months after stopping high-dose steroids. For inhaled steroids: no evidence of immunosuppression at recommended doses — not a contraindication to live vaccines.
🚨 Anaphylaxis to a Vaccine: What to Do
- Treat immediately: IM adrenaline 500mcg (adults and children >12 years); call 999; standard RCUK 2021 algorithm
- Report: Report to MHRA Yellow Card scheme
- Contraindicate: The implicated vaccine (and potentially related vaccines containing the same component) is now contraindicated pending specialist review
- Refer to allergy/immunology clinic: For investigation of the responsible component (e.g., gelatin, neomycin, latex from vial stopper, polyethylene glycol, yeast) and risk assessment for future vaccination needs
- Do NOT assume that the patient cannot receive any vaccines in future — specialist assessment determines which vaccines are safe
💊 Common Vaccine Components Causing Allergy
| Component | Vaccines Potentially Affected | Notes |
|---|---|---|
| Egg protein (ovalbumin) | Influenza, yellow fever | Mild egg allergy: OK in primary care. Anaphylaxis to egg: specialist setting for influenza. MMR: safe regardless. |
| Gelatin | MMR, varicella, some flu vaccines | Rare cause of anaphylaxis. Halal/kosher concerns may arise. |
| Neomycin | MMR, varicella, IPV | Contact allergy (patch test positive) is NOT a contraindication. Anaphylaxis to neomycin is a contraindication. |
| Polyethylene glycol (PEG) | mRNA COVID-19 vaccines (Pfizer, Moderna) | Rare. Specialist assessment required if confirmed allergy. |
| Yeast | Hepatitis B, HPV, meningococcal B | Yeast allergy (e.g., to bread/beer) is NOT usually a contraindication unless severe anaphylaxis to yeast documented. |
Source: UKHSA Green Book (various chapters). Seek specialist advice for any suspected vaccine-related anaphylaxis before repeat dosing.
💬 Patient Communication Tip
“Having an allergy — even a serious one — does not usually mean you cannot be vaccinated. The vast majority of allergic patients receive vaccines safely and on schedule. If there’s a genuine concern about a specific component, we can usually find a safe approach with specialist advice.”
💪 You've Got This!
Allergy & Immunology: From Sniffles to Life-Saving Adrenaline
You've Mastered the Essentials
From allergic reaction types and terminology to anaphylaxis management — you have the frameworks to confidently assess and manage allergic presentations in primary care.
You Know the Red Flags
Anaphylaxis, airway-threatening angioedema, DRESS, SJS/TEN — you can recognise life-threatening presentations and know when to act fast and when to refer.
You Can Navigate Vaccines
Egg allergy, previous reactions, immunocompromised patients — you know what is and isn’t a true contraindication, and when to seek specialist advice.
🎯 Core Take-Home Messages
Now go forth and conquer those allergy consultations. You've got this! 💪
Sources: NICE CKS; NICE CG134; RCUK 2021; BSACI; UKHSA Green Book; NICE ESUOM31