Allergies

NCERT grounding

Section 7.2.5 Allergies of NCERT Class XII Biology defines allergy as the exaggerated response of the immune system to certain antigens present in the environment. The substances that elicit such a response are called allergens. The antibodies produced against allergens are of the IgE class, and the symptoms arise from histamine and serotonin released by mast cells. Common allergens listed by name are mites in dust, pollens and animal dander; common symptoms listed are sneezing, watery eyes, running nose and difficulty in breathing. The textbook also notes that modern lifestyle and a protected early-life environment have lowered immunity and increased sensitivity, with metro-city children showing more allergies and asthma.

"The exaggerated response of the immune system to certain antigens present in the environment is called allergy."

NCERT Class XII Biology · §7.2.5

Mechanism of an allergic reaction

An allergic reaction is a misdirected version of the same humoral pathway used against parasites. The body has been engineered to mount IgE responses against worms and other large parasites; in allergy that machinery fires against pollen, dust mites or peanut protein instead. The reaction unfolds in two distinct visits — a silent sensitisation phase on first exposure and an explosive effector phase on every subsequent exposure.

On first exposure, the allergen is captured by antigen-presenting cells, processed and shown to helper T-cells. In allergy-prone individuals these T-cells skew toward a Th2 profile, secreting interleukins that drive nearby B-cells to class-switch their immunoglobulin output to IgE. The newly minted IgE antibodies leave the lymph node and travel through blood and tissue fluid, finally docking onto high-affinity Fcε receptors on the surface of mast cells (in connective tissue) and basophils (circulating). The person now feels nothing, but every mucosal surface — nose, conjunctiva, bronchi, gut — is studded with allergen-specific IgE.

On re-exposure, allergen molecules diffusing across the mucosa bridge two adjacent surface IgE molecules. This cross-linking is the trigger: it deforms the underlying receptors, opens calcium channels, and causes the mast cell to degranulate — emptying its pre-formed granules into the surrounding tissue within seconds. The granules contain histamine and serotonin (the two chemicals NCERT names) plus heparin, tryptase and chemotactic factors. Histamine produces the dominant clinical picture: arteriolar vasodilation (redness, warmth), increased capillary permeability (oedema, runny nose, watery eyes), bronchial smooth-muscle contraction (wheeze, difficulty in breathing) and stimulation of mucus glands.

The whole sequence from allergen contact to visible symptom can be under a minute, which is why allergic reactions feel sudden. A delayed late-phase response, driven by eosinophils and other mediators recruited by chemotactic factors, contributes the prolonged congestion and bronchial hyper-responsiveness that follow an acute episode by several hours.

Common allergens and symptoms

NCERT names three allergens explicitly: mites in dust, pollens and animal dander. The chapter opening also recalls a familiar scenario — a person who travels to a new place, starts sneezing and wheezing for no apparent reason, then recovers when they leave — to illustrate environmental dependence. For NEET, that opening list (plus a one-line set of symptoms) is the load-bearing content.

The clinical picture is short and stereotyped. NCERT lists sneezing, watery eyes, running nose and difficulty in breathing. At the airway level, histamine constricts bronchial smooth muscle and increases mucus secretion — the link to asthma. At the skin level it dilates capillaries to produce hives and itch. In severe systemic reactions (insect venom, food, penicillin), mediators flood the circulation and cause anaphylaxis: a sudden fall in blood pressure, laryngeal oedema and bronchospasm, fatal if untreated within minutes.

Hypersensitivity classes at NEET level

NEET does not require Gell and Coombs nomenclature, but a NEET aspirant should recognise that allergy as defined by NCERT is a Type I (immediate, IgE-mediated) hypersensitivity. The wider classification is worth one minute of revision because it tells you which other clinical entities in the chapter do not count as allergy in the strict sense.

Pharmacology: antihistamines, adrenaline, steroids

NCERT names three drug groups that "quickly reduce the symptoms of allergy". Each targets a different step of the cascade, and exam questions sometimes test which drug is appropriate to which scenario.

Antihistamines are competitive antagonists at the H1 histamine receptor; they stop new histamine molecules from acting on smooth muscle, capillaries and sensory nerves. They are the routine drug for hay fever, hives and mild rhinitis. Adrenaline (epinephrine) is the emergency drug for anaphylaxis — it produces vasoconstriction (raising blood pressure), bronchodilation (opening airways) and stabilises the mast cell itself, reversing the entire picture within minutes. Steroids (corticosteroids such as prednisolone, or inhaled budesonide) suppress the broader inflammatory cascade — they take hours to act but control the late-phase response and chronic conditions like asthma and allergic rhinitis.

Rising prevalence and the hygiene hypothesis

NCERT closes §7.2.5 with a clinically loaded sentence: "more and more children in metro cities of India suffer from allergies and asthma due to sensitivity to the environment. This could be because of the protected environment provided early in life." This is the textbook's nod to the hygiene hypothesis — the idea that the immune system, deprived of microbial challenges in infancy, fails to learn tolerance and skews toward allergic Th2 responses against harmless antigens.

Worked examples

Common confusion & NEET traps