Which of the following mediators, pre-formed in mast cell granules, is primarily responsible for the immediate vasodilation and increased vascular permeability seen in Type I hypersensitivity?
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Correct answer: Histamine release
Histamine is a pre-formed vasoactive amine that acts rapidly on H1 receptors to cause smooth muscle contraction and increased capillary permeability. Leukotrienes and prostaglandins are synthesized de novo and contribute more significantly to the late-phase response.
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