Which mediator is commonly released during allergic reactions and contributes to vasodilation and increased vascular permeability?

• A. Epinephrine
• B. Histamine
• C. Leukotrienes
• D. Sepsis

Answer: (B)

When thinking about what causes the rapid redness, warmth, and swelling in an allergic reaction, focus on the chemical messenger released from mast cells and basophils. The mediator responsible for both vasodilation and increased vascular permeability is histamine. Once released, histamine binds to receptors on endothelial cells, making the cell junctions more permeable and allowing fluid to leak into the surrounding tissue. This leakage produces swelling, and the increased blood flow from vasodilation contributes to the redness and warmth.

Histamine is stored in granules inside mast cells and basophils and is released quickly after an allergen cross-links IgE on their surfaces, driving the immediate phase of the allergic response. While leukotrienes also promote permeability and inflammation, histamine is the primary mediator for these early vascular changes. Epinephrine can counteract many allergic symptoms through vasoconstriction and bronchodilation, and sepsis involves a broader inflammatory response rather than a single mediating molecule.