[Significance of histamine in immediate allergic reactions]

Histamine has generally been accepted to be an important mediator of immediate-type-allergic reactions. Histamine is released from antigen-induced sensitized mast cells and basophils. Histamine acts on its effector cells via specific H1-and H2-receptors. The stimulation of the H1-receptor type results in a contraction of smooth muscle cells of the gastrointestinal tract, the bronchioli and the large blood vessels. In addition, there are increased vascular permeability and dilation of small blood vessels. Activation of nasal exocrine glands is also due to the H1-receptor. The main effects of H2-receptor activation results in gastric acid secretion and dilation of small blood vessels. The effects of histamine are inhibited by specific H1-and H2-antagonists.