Histamine is an active substance found in a large range of living organisms that plays a major role in allergic reaction, dilating blood vessels and increasing the permeability of vessel walls. It also acts as a neurotransmitter or chemical messenger that carries signals from one nerve to another, as well as performing several other important functions in various bodily tissues.
Histamine is classified as an amine because it is based on the structure of ammonia and is formed through decarboxylation of the amino acid histidine. Histamine is an autacoid, which means it acts similarly to a local hormone, near its site of synthesis. It is produced as part of the local immune response to invading bodies and triggers inflammation.
Histamine exerts its effects by binding to histamine receptors on cells’ surfaces. There are four types of histamine receptor: H1, H2, H3 and H4. The binding of histamine to these receptors stimulates them to produce functional responses and these are described in more detail below