Histamine (HA)

Histamine is the most important mediator and is mostly found in the initial phase of an anaphylactic reaction ("immediate type" allergy). Histamine is developed by the enzymatic decarboxylation of histidine. In the organism, histamine is present in nearly all tissues, and it is mainly stored in the metachromatic granula of mast cells and the basophilic leukocytes.

It is present in an inactive bound form and is only released as required. Histamine acts predominantly on smooth muscle and blood vessels.It is responsible for the bronchoconstriction occurring during the acute phase. In the vessels, its constrictive effect is limited to the venula, whereas arterioles are dilated. Furthermore, histamine causes a contraction of the cells of the vascular endothelium and increases the vascular permeability, thereby allowing higher-molecular substances to escape into the tissue.