Which cells respond to early immunologic challenges and secrete vasoactive substances?

• A. Eosinophils
• B. Plasma cells
• C. Mast cells
• D. Fibroblasts

Answer: (C)

Mast cells respond to early immunologic challenges by releasing vasoactive substances. Located in connective tissue near blood vessels and mucosal surfaces, they’re primed to react when IgE on their surface is cross‑linked by an allergen or other stimuli. This degranulation unleashes histamine as the primary vasoactive mediator, causing vasodilation and increased vascular permeability that produce redness, swelling, and edema. Additional mediators like leukotrienes and prostaglandins amplify these vascular changes and help recruit other immune cells, shaping the rapid, early phase of the response. Other cell types in the options have different roles: eosinophils handle later stages and parasite defense, plasma cells make antibodies, and fibroblasts build extracellular matrix—none of which are the immediate carriers of vasoactive mediators in this context.