Question
Explain the mechanism of an allergic reaction (Type I hypersensitivity). What role does IgE play? Why do symptoms appear on second exposure to the allergen, not the first? Name the chemicals released by mast cells during an allergic response.
(NEET pattern — mechanism + reasoning)
Solution — Step by Step
When an allergen (pollen, dust mite, peanut protein) enters the body for the first time, B cells produce IgE antibodies specific to that allergen. These IgE molecules bind to the surface of mast cells (found in skin, respiratory tract, and gut) and basophils in blood. This is called sensitization.
No symptoms appear during first exposure — the immune system is just “learning” and preparing.
On re-exposure, the same allergen binds to the IgE molecules already sitting on mast cell surfaces. When an allergen molecule cross-links two adjacent IgE molecules, the mast cell is triggered to degranulate — it releases a cocktail of chemicals from its granules.
| Chemical | Effect |
|---|---|
| Histamine | Vasodilation, increased permeability, itching, bronchoconstriction |
| Serotonin | Smooth muscle contraction |
| Prostaglandins | Inflammation, pain |
| Leukotrienes | Sustained bronchoconstriction (worse than histamine) |
| Heparin | Anticoagulant (local) |
These chemicals cause the classic allergy symptoms: sneezing, watery eyes, skin rash, swelling, and in severe cases, anaphylaxis (life-threatening drop in blood pressure and airway closure).
Antihistamines block histamine receptors and relieve symptoms. Epinephrine (adrenaline) is used for anaphylactic shock — it reverses vasodilation and bronchospasm. Steroids reduce inflammation in chronic allergies.
graph TD
A["Allergen — First Exposure"] --> B["B cells produce IgE"]
B --> C["IgE binds to Mast Cells"]
C --> D["Sensitization Complete"]
D --> E["Allergen — Second Exposure"]
E --> F["Allergen cross-links IgE on mast cells"]
F --> G["Mast Cell Degranulation"]
G --> H["Histamine, Serotonin, Leukotrienes released"]
H --> I["Allergy Symptoms"]
style A fill:#fbbf24,stroke:#000,stroke-width:2px
style G fill:#fca5a5,stroke:#000,stroke-width:2px
style I fill:#f9a8d4,stroke:#000Why This Works
Allergy is essentially an overreaction of the immune system to a harmless substance. The IgE class of antibodies evolved to fight parasitic worm infections — they trigger mast cell degranulation to release chemicals that attack large parasites. In allergic individuals, this same pathway fires against pollen or food proteins instead.
The reason symptoms do not appear on first exposure is that the immune system needs time to produce IgE and coat mast cells with it. Only after this sensitization phase is the “trigger” set. Second exposure pulls that trigger.
Common Mistake
Students commonly write that allergy symptoms appear on “first exposure.” This is incorrect. The first exposure causes sensitization (IgE production), which is silent. Symptoms only appear on second or subsequent exposures when the allergen encounters pre-formed IgE on mast cells. This is a classic NEET question trap.
Remember: the antibody involved in allergy is IgE (E for “exaggeration” — an exaggerated immune response). IgE is the least abundant antibody in blood but most potent in triggering allergic responses. NEET often asks which immunoglobulin class is involved in allergy — the answer is always IgE.