Postcapillary venules, gaps

Increased permeability results mainly from actions of histamine on postcapillary venules, where histamine causes the endothelial cells to contract and separate at their boundaries and thus to expose the basement membrane, which is freely permeable to plasma protein and fluid. The gaps between endothelial cells also may permit passage of circulating cells that are recruited to the tissues during the mast cell response. Recruitment of circulating leukocytes is promoted by Hj-receptor-mediated up-regulation of leukocyte adhesion. This process involves histamine-induced expression of the adhesion molecule P-selectin on the endothelial cells. 

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