Which mediator is most directly responsible for increasing capillary permeability during inflammation?

• A. Histamine
• B. Bradykinin
• C. Lymphocytes
• D. Platelets

Answer: (A)

When tissue injury triggers acute inflammation, the immediate change is the rapid, direct increase in capillary permeability driven by histamine. Histamine is released from mast cells and basophils and acts on endothelial cells lining the postcapillary venules. Binding to H1 receptors on these cells causes their cytoskeleton to contract, creating gaps between adjacent endothelial cells. Those gaps allow plasma fluid and proteins to leak into the surrounding tissue, producing the characteristic edema of acute inflammation. Bradykinin also raises permeability but tends to act more slowly and is more closely tied to pain and vasodilation, whereas lymphocytes are immune cells not responsible for the quick permeability change, and platelets contribute other mediators but histamine is the primary, fastest-acting driver of this vascular leak.