Histamine is a chemical produced by the body that aids in immune response and acts as a neurotransmitter. In response to foreign pathogens in the body, this chemical is produced by basophils, a type of white blood cell, and mast cells, cells in the connective tissue with similar characteristics to basophils. Histamine helps fight off infection by making capillaries more permeable to white blood cells that fight pathogens.
Four types of histamine receptors, which interact with released histamine to produce a reaction, have been discovered in the body. H1 receptors are found on the smooth muscle tissue of the internal organs, the endothelium lining blood vessels, and central nervous system tissue. The interaction of histamine with these receptors is responsible for hives, itching and swelling due to insect bites and similar allergic reactions, and allergic rhinitis, or cold-like symptoms due to allergic reaction. H2 receptors are located on the parietal cells on the stomach lining and stimulate the secretion of gastric acid when activated; this process is a normal part of biological function and not a response to pathogens.
H3 receptors are located in the tissue of the central and parietal nervous system and are responsible for the decreased release of neurotransmitters including acetylcholine, histamine, norepinephrine, and serotonin. H4 receptors are located in the basophils, bone marrow, thymus, small intestine, spleen, and colon. They play a role in chemotaxis, the movement of body cells in reaction to a chemical in their environment.
In addition to its role in immune response, histamine helps regulate a number of processes in the body. It aids in the digestive function of the stomach, as mentioned above, and helps produce an orgasm through mast cells in the genitals. Histamine also helps regulate sleep, as the body produces more upon waking and less as the sleep cycle proceeds. For this reason, antihistamines can help a person fall asleep by limiting the chemical's release.