Histamine is a substance, with the chemical formula C₃H₃N₂(CH₂)₂NH₂, which is derived from the amino acid histidine. It occurs naturally in many plants and in animal tissues, and is released when a tissue is injured. Histamine is involved in many allergic reactions, such as hay fever, which can be treated with antihistamine drugs. It also stimulates the gastric secretion of hydrochloric acid. Melting point 83°C.

 

Histamine and the allergic response

Allergies come about when the immune system responds to a normally harmless substance, such as pollen or dust, that comes in contact with lymphocytes specific for that antigen. Often, the lymphocyte triggered to respond is a mast cell. For the response to take place, free-floating immunoglobulin E (IgE) molecules specific to the antigen must first be attached to cell surface receptors on the mast cells. Antigen binding to the mast cell-attached IgE then triggers the mast cell to respond. This response often includes the release of histamine, which may then contribute to two main allergic symptoms: inflammation and contraction of smooth muscle.

 

Histamine can cause inflammation directly as well as indirectly. Upon release of histamine by an antigen activated mast cell, the permeability of vessels near the site is increased. This happens because histamine induces phosphorylation of an intercellular adhesion protein found on vascular endothelial cells. (For this reason, histamine is said to be vasoactive.) The phosphorylation creates gaps between the cells in vascular tissue through which blood fluids, including leukocytes (white blood cells), can enter the extracellular space and cause swelling. Indirectly, histamine contributes to inflammation by affecting the functions of other leukocytes in the area.

 

Histamine's second type of allergic response is one of the major causes of asthma. In response to an allergen – a substance that triggers an allergic reaction – histamine, along with other chemicals, causes the contraction of smooth muscle. Consequently, the muscles surrounding the airways constrict causing shortness of breath and, in extreme cases, complete closure of the trachea.