These are those agents or drugs or chemical substances which inhibits the action of histamine in human body.
Antihistamines are drug used to treat allergies, allergic rhinitis, the common cold, the flu, and other disorders.
Histamine
A chemical that cells release during allergic and inflammatory reactions, as well as in response to injury which causes smooth muscle contraction and vascular constriction.
Histamine = hist + amine. Hist word means “tissue” and amine means “NH2”. So, it is an amine compound that is releases by tissue.
It is a biological substance that is released by basophils and mast cells as a local response against parasite infection or to protect cells from any foreign substance.
Structure-
Biosynthesis of histamine
Histamine is synthesised by decarboxylation of histidine which is found in all organs and tissue of the human body. This synthesis is done in presence of catalyst enzyme L-histidine decarboxylase.
Storage of histamine
Most of the body's histamine is produced in granules by basophils and mast cells, these are the type of white blood cells (leukocytes).
Histamine (non- mast cell) is also present in a variety of tissues, including the brain's hypothalamus, where it works as a neurotransmitter.
The enterochromaffin-like (ECL) cell of the stomach is another important location for histamine storage and release.
Usually histamine is present in whole body, but its concentration is high in skin, mucosal cell of lungs, intestine, urinary tract.
Its concentration is also high in mammalian cerebro spinal fluid (brain).
Release of Histamine-
Histamine is released by the interaction of antigen with IgE antibodies on the surface of mast cell.
The antigen are protein and polysaccharides obtained from various source like dust, pollen gains, food stuff, venoms, toxins, detergents, and some other chemicals etc.,
There are some other inflammatory mediators release such as prostaglandins, kinin and platelets activating factor.
After the release of histamine from mast cell, it performs following physiological effects-
Hypersensitivity like itching, sneezing, watery eye and running nose.
contracts smooth muscles of lungs
inflammation which causes pain
allergic reactions
bronchospasm (hard to breath), cause due to inflammation and allergic reactions in the airways.
Vasodilation, cause hypotension
Gastric secretion, increase HCL secretion causes Acidity etc.,
Catabolism or degradation of histamine-
Diamine oxidase (DAO) and histamine-N-methyltransferase (HNMT) are mainly responsible for the degradation of histamine (HNMT)
Histamine inactivates by many reactions and enzymes which change the structure of histamine, after changing structure, histamine does not bind with receptor due to changes its structure.
