Which statement about Type I hypersensitivity is true?

Which statement about Type I hypersensitivity is true?

• A. It is mediated by IgE binding and mast cell degranulation.
• B. It is mediated by immune complexes in tissues.
• C. It is a delayed hypersensitivity reaction.
• D. Epinephrine is ineffective in treatment.

Answer: (A)

Type I hypersensitivity is an immediate, IgE-mediated reaction. Sensitized individuals produce IgE against an allergen, and IgE binds to high-affinity receptors on mast cells and basophils. On re-exposure, the allergen cross-links these bound IgE molecules, triggering rapid degranulation and release of mediators such as histamine, leukotrienes, and prostaglandins. This leads to vasodilation, increased vascular permeability, and smooth muscle contraction, producing symptoms ranging from hives and runny nose to bronchospasm and, in severe cases, anaphylaxis. This mechanism is distinct from the other hypersensitivity types: Type III involves immune complex deposition; Type IV is a delayed T-cell–mediated reaction. Epinephrine, not ineffective, is the primary treatment for anaphylaxis caused by this reaction.

Type I hypersensitivity is an immediate, IgE-mediated reaction. Sensitized individuals produce IgE against an allergen, and IgE binds to high-affinity receptors on mast cells and basophils. On re-exposure, the allergen cross-links these bound IgE molecules, triggering rapid degranulation and release of mediators such as histamine, leukotrienes, and prostaglandins. This leads to vasodilation, increased vascular permeability, and smooth muscle contraction, producing symptoms ranging from hives and runny nose to bronchospasm and, in severe cases, anaphylaxis. This mechanism is distinct from the other hypersensitivity types: Type III involves immune complex deposition; Type IV is a delayed T-cell–mediated reaction. Epinephrine, not ineffective, is the primary treatment for anaphylaxis caused by this reaction.