Answer: Hypersensitivity reaction where the immune system responds excessively to a harmless antigen (allergen).
- A A normal, appropriately calibrated immune response that the body mounts against a genuine invading pathogen
- B Hypersensitivity reaction where the immune system responds excessively to a harmless antigen (allergen)
- C A congenital deficiency of circulating T lymphocytes that has been present in the bloodstream since birth
- D A type of autoimmune disease in which antibodies mistakenly target and damage the body's own healthy cells
Correct answer: B. Hypersensitivity reaction where the immune system responds excessively to a harmless antigen (allergen)
Explanation: Allergy is IgE-mediated hypersensitivity to allergens (dust, pollen, food, etc.). IgE binds to mast cells; allergen crosslinks IgE, causing mast cells to release histamine and other chemicals, causing allergy symptoms.
An antibody is Y-shaped, with two identical antigen-binding sites at the tips of the variable regions - each shaped to recognise one specific antigen - while the constant (Fc) region at the base is recognised by immune cells to trigger further responses.
Concept context
Innate and adaptive immunity, B cells, T cells, antibodies, vaccines, and immune disorders. Critical for NEET.