Histamine

It is a potent tissue amine widely distributed in plant and animal tissues and in the venoms of bees. In man, it is formed by decarboxylation of histidine and major portion is stored in mast cells and basophils.

Mechanisms of Action: It acts on 2 major types of receptors

1. Stimulation of H₁ receptors results in smooth muscle contraction, increased vascular permeability, and mucus production. These effects are blocked competitively by H₁ antagonists.
2. Activation of H₂ receptors increases gastric acid production, and this effect is blocked by H₂ blockers such as cimetidine.

Both types of receptors are involved in vascular dilatation and edema formation.