Question
A student develops sneezing, watery eyes, and skin rashes every spring when pollen count rises. Explain the immunological mechanism behind this allergic response, identifying the antibody class involved and the cells that release histamine.
Allergy Response Pathway
flowchart TD
A["First Exposure to Allergen (e.g., Pollen)"] --> B["B-cells produce IgE antibodies"]
B --> C["IgE binds to surface of Mast Cells"]
C --> D["Sensitisation complete — no symptoms yet"]
D --> E["Second Exposure to Same Allergen"]
E --> F["Allergen cross-links IgE on Mast Cells"]
F --> G["Mast Cell Degranulation"]
G --> H["Histamine release"]
G --> I["Serotonin release"]
G --> J["Prostaglandins release"]
H --> K["Vasodilation, increased permeability"]
K --> L["Sneezing, runny nose, rashes, swelling"]Solution — Step by Step
When pollen enters the body for the first time, the immune system mistakenly identifies it as a threat. B-lymphocytes produce IgE antibodies specific to that allergen. These IgE molecules attach to the surface of mast cells (found in connective tissue, especially near skin, lungs, and nasal passages). No symptoms appear yet — this is the sensitisation phase.
On re-exposure, the same allergen binds to the IgE already sitting on mast cells. When an allergen molecule cross-links two adjacent IgE molecules, it triggers mast cell degranulation — the cell releases its granules containing histamine, serotonin, and prostaglandins.
Histamine is the main culprit. It causes:
- Vasodilation — blood vessels widen, causing redness and warmth
- Increased vascular permeability — fluid leaks into tissues, causing swelling (oedema)
- Smooth muscle contraction — in bronchi, this causes difficulty breathing (asthma)
- Mucus secretion — sneezing and runny nose
This is why antihistamines (like cetirizine) work — they block histamine receptors and reduce symptoms.
Why This Works
Allergy is essentially an exaggerated immune response to a harmless substance. The immune system has evolved to fight parasites using IgE — but in allergic individuals, it misfires against pollen, dust mites, or food proteins.
The reason symptoms appear only on the second exposure is that the first exposure is needed to produce IgE and “arm” the mast cells. This is why a person who has never been exposed to a particular pollen will not show allergic symptoms on first contact.
Remember the antibody hierarchy for NEET: IgE = allergy, IgG = most abundant in blood (crosses placenta), IgA = mucosal immunity (saliva, tears, breast milk), IgM = first responder (primary immune response), IgD = B-cell surface receptor.
Alternative Method
You can also think of this through the Type I Hypersensitivity classification:
- Type I (Immediate) — IgE-mediated, mast cell degranulation (allergy, anaphylaxis)
- Type II (Cytotoxic) — IgG/IgM against cell surface antigens
- Type III (Immune complex) — antigen-antibody complexes deposit in tissues
- Type IV (Delayed) — T-cell mediated (contact dermatitis, TB skin test)
NEET primarily tests Type I, but knowing all four helps in elimination-based answering.
Common Mistake
Students often write that antibodies cause allergy without specifying which class. NEET expects you to name IgE specifically. Writing just “antibodies” or “IgG” will cost you marks. Also, the cells that release histamine are mast cells — not basophils (though basophils also carry IgE, mast cells are the primary players in tissue-level allergic responses).