Allergy mediators — Which chemical is typically released by the body during an acute allergic (type I hypersensitivity) reaction?

Introduction:
Type I hypersensitivity (immediate allergy) involves IgE-sensitized mast cells and basophils that rapidly degranulate upon allergen cross-linking. Understanding the key mediators helps explain clinical signs such as itching, wheal-and-flare, bronchospasm, and hypotension.

Given Data / Assumptions:

• Mast cells/basophils carry surface-bound IgE via FcεRI.
• Cross-linking by allergen triggers degranulation.
• Granules and lipid mediators cause vasodilation, increased permeability, and smooth muscle effects.

Concept / Approach:
Identify the canonical mediator stored preformed in granules: histamine. Distinguish histamine from external allergens, therapeutic antihistamines, cytotoxic proteins like perforin, and complement enzymes, which are not the immediate mast-cell mediator in classic allergy.

Step-by-Step Solution:

Verification / Alternative check:
Clinical efficacy of H1 antihistamines in urticaria and allergic rhinitis confirms histamine’s pivotal role in early-phase symptoms.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing triggers with mediators; attributing cytotoxic granule proteins to allergic mast-cell responses.

Final Answer:
Histamine.