By Dr Ananya Mandal, MD
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.
Much of the histamine in the body is produced by the granules in mast cells and basophils, as part of a local immune response to the presence of invading bodies. The basophils and mast cells are found in nearby connective tissue and are particularly abundant at sites where injury may occur such as the nose, mouth, feet, surfaces of the internal organs and blood vessels. Histamine that is not generated by mast cells is made in the brain, where it acts as a neurotransmitter. Another important site where histamine is stored and released is the enterochromaffin-like (ECL) cells of the stomach.
The main purpose of histamine release from mast cells and basophils is to trigger inflammation. When these cells are sensitized by immunoglobulin E (IgE) attached to their membranes, the granules inside them break down to release histamine in the presence of the appropriate antigen. Sometimes, exposure to certain drugs such as curare alkaloids or morphine can displace histamine inside granules, leading to its release. Another agent that causes the release of histamine is the antibiotic polymyxin.
Reviewed by Sally Robertson, BSc
Last Updated: Aug 19, 2014