Which physiologic effect is primarily caused by histamine during an allergic reaction?

• A. Vasodilation and increased vascular permeability
• B. Stimulation of insulin release
• C. Inhibition of platelet aggregation
• D. Activation of platelet production

Answer: (A)

Histamine released during an allergic reaction mainly drives changes in blood vessels: it causes vasodilation and increased vascular permeability. When histamine binds to receptors on the endothelium, it promotes relaxation of nearby smooth muscle and nitric oxide production, widening the small blood vessels. At the same time, it makes the endothelial cells contract slightly, creating gaps that let fluid and proteins leak into surrounding tissue. This combination produces redness, warmth, and edema—the hallmark wheal-and-flare response of an allergic reaction.

Other effects listed aren’t the primary features of an allergic response: histamine isn’t the main driver of insulin release, and it isn’t primarily responsible for inhibiting platelet aggregation or promoting platelet production.