Mast cells, which dwell in tissues and function like basophils, act during an inflammatory response. They release histamine and other chemicals into interstatial fluid. Their chemical signals trigger vasodilation of arterioles that thread thgough the damaged tissue. Vasodilation, remember, is an increase in a vessel's diameter after smooth muscle in its wall has relaxed. When arterioles become engorged with blood, the affected tissue reddens and gets warmer, owing to blood-borne metabolic heat.
Released histamine also increases the permeability of the thin-walled capillaries in the tissue. It induces the endothelial cells making up the capillary wall to pill apart farther at the narrow clefts between them. This makes the capillaries abnormally "leaky."