Q.In which of the following conditions/diseases is there a substantial increase in the activity of mast cells observed in the human body? (A) Typhoid (B) Allergy (C) Ascariasis (D) AIDS

The key idea is that mast cells are central to immediate hypersensitivity reactions — they release histamine and other mediators when triggered by allergens.

• Mast cells are not significantly involved in typhoid (bacterial infection, mainly macrophage response) or AIDS (viral depletion of T-cells).
• Ascariasis (a helminth infection) does involve mast cells and eosinophils, but the most dramatic and characteristic increase in mast cell activity is seen in allergic reactions. …

The core concept: mast cells are the body's "alarm cells" for immediate hypersensitivity.

Mast cells reside in connective tissues, especially near blood vessels and mucosal surfaces. They are packed with granules containing histamine, heparin, and other inflammatory mediators. When an allergen (like pollen or dust) cross-links IgE antibodies bound to the mast cell surface, the cell degranulates explosively — releasing histamine and causing the classic symptoms of allergy: swelling, itching, redness, and bronchoconstriction. This is a type I hypersensitivity reaction.

Now, let's examine each option against this function:

1. Typhoid — Caused by Salmonella typhi bacteria. The immune response here is primarily cell-mediated (T-cells) and antibody-mediated (IgG, IgM). Macrophages and neutrophils are the key players. Mast cells are not substantially involved; their activity does not increase significantly.

2. Allergy — This is the hallmark condition for mast cell activation. In an allergic individual, mast cells are sensitized by IgE antibodies specific to harmless environmental antigens. On re-exposure, the allergen binds to these surface IgE molecules, triggering rapid degranulation. This is the only condition among the options where mast cell activity is both substantial and diagnostic.

3. Ascariasis — A helminthic infection caused by Ascaris lumbricoides. The immune response against large parasites involves eosinophils (which are recruited to attack the worm), Th2 cells, and IgE production. While mast cells can participate in anti-parasite responses, their increase is not substantial compared to the eosinophil-dominated reaction. The primary effector cells here are eosinophils, not mast cells. …