Histamine is an amine that is produced as part of a local immune response to cause inflammation. It also performs several important functions in the bowel and acts as a neurotransmitter or chemical messenger that carries signals from one nerve to another.
Histamine is secreted by basophils and mast cells as part of a local immune response to the presence of invading bodies. The basophils and mast cells are found in nearby connective tissue. This histamine release causes capillaries to become more permeable to white blood cells and other proteins, which proceed to target and attack foreign bodies in the affected tissue. Aside from humans, histamine is found in virtually all animals.
Histamine was first synthesized in 1907 and its pharmacological properties were demonstrated in 1911. Because the substance was extracted from tissue, the word “histo” was used to describe this “amine”. Receptor subtypes for histamine were characterized in 1966 and the first antihistamine drugs were developed between 1943 and 1944.
Histamine gives a colorless hygroscopic crystal that melts at 84°C and is easily dissolved in water or ethanol. It is insoluble in ether. In a solution of water, histamine exists in two tautomeric forms, ''Nπ-H''-histamine and ''Nτ-H''-histamine. Its chemical structure includes two basic centres – one of them is an aliphatic amino group and the other is whichever nitrogen in the imidazole does not have a proton. In the body, the aliphatic amino group (having a pKa around 9.4) is protonated and the nitrogen a (pKa ≈ 5.8) is not protonated.
For histamine to form, the amino acid histidine undergoes decarboxylation. This chemical reaction is catalyzed by the enzyme L-histidine decarboxylase. Histamine is a hydrophilic vasoactive amine and once formed, it is either quickly inactivated or stored. When released at synapses, it is broken down by acetaldehyde dehydrogenase. When this enzyme is deficient, there is an increased risk of allergic reactions, as histamine accumulates in the synapses. Histamine is broken down by the enzymes diamine oxidase and histamine-N-methyltransferase.