Types of Allergic Reaction
| Type | Timing | Mechanism | Example |
|---|---|---|---|
| IgE-mediated (immediate) | Minutes | Mast cell degranulation via IgE | Nut allergy, bee sting, latex |
| Non-IgE mediated | Hours–days | T-cell mediated | Contact dermatitis, coeliac (partly) |
| Mixed IgE/non-IgE | Variable | Both mechanisms | Atopic eczema |
| Food intolerance (NOT allergy) | Variable | Not immune-mediated | Lactose intolerance, IBS triggers |
Allergy Blood Tests (Specific IgE)
| Test | What It Measures | Interpretation |
|---|---|---|
| Total IgE | Overall allergy burden | High in atopy; not diagnostic alone |
| Specific IgE (RAST) | IgE antibodies to a specific allergen | Class 0 (negative) to Class 6 (very high); does NOT confirm clinical allergy alone |
| Component resolved diagnosis (CRD) | IgE to specific proteins within an allergen | Distinguishes mild from severe peanut allergy (Ara h 2) |
| Tryptase | Mast cell marker — rises after anaphylaxis | Useful within 1–6 hours of suspected anaphylaxis; high at baseline suggests mastocytosis |
Positive IgE ≠ Clinical AllergyA positive specific IgE test means sensitisation — the immune system has made antibodies. But 50% of sensitised people have no clinical reaction. Allergy diagnosis requires: positive test PLUS history of symptoms on exposure.
Anaphylaxis — Emergency Action
- Call 999 / 911 immediately
- Administer adrenaline (epinephrine) auto-injector (EpiPen, Jext) into outer thigh — through clothing
- Lay flat with legs raised (or sit upright if breathless)
- Second adrenaline dose after 5 minutes if no improvement
- Observe in hospital for minimum 6–12 hours (biphasic reaction risk)
Allergy Treatments
| Treatment | Used For | Evidence |
|---|---|---|
| Avoidance | All allergies | Cornerstone — requires education on hidden sources |
| Antihistamines | Mild allergic reactions, rhinitis, urticaria | Effective for mild symptoms |
| Nasal steroids | Allergic rhinitis | Most effective for hay fever |
| Sublingual immunotherapy | Grass pollen, house dust mite, some foods | Changes immune response over 3 years — can 'cure' |
| Subcutaneous immunotherapy | Wasp/bee venom allergy, pollen | 90% effective for venom allergy |
| Oral immunotherapy (OIT) | Peanut, milk allergy | Specialist centres — desensitises but doesn't cure |
What is the difference between allergy and intolerance?
Allergy involves the immune system (IgE antibodies or T-cells) and can cause anaphylaxis. Intolerance (e.g. lactose intolerance) is a non-immune mechanism and is not life-threatening.
Are home allergy test kits reliable?
Most over-the-counter IgE home tests have poor standardisation and are unreliable. They can cause unnecessary avoidance and anxiety. Proper allergy testing should be done through an allergist or immunologist.
Can allergies develop in adulthood?
Yes — new allergies can develop at any age, including late adulthood. Adult-onset food allergy (particularly shellfish, tree nuts, fish) is well recognised and may be triggered by loss of oral tolerance.
Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.