- Its mechanism of action is to exert its influence by binding to histamine receptors, which are all GPCR. They may increase intracellular cAMP or intracellular Ca2+.
- Appears to play a role in
- Immediate allergic reactions and anaphylaxis
- Gastric acid secretion
- Functions as a neurotransmitter/neuromodulator
- Has a role in immune modulation/chemotaxis of WBCs
- Tissue and organ effects as follows:
- Immune system
- Causes vasodilation and increased permeability of vessels.
- Chemoattractant for WBCs and neutrophils.
- Histamine inhibits the release of lysosome contents, and inhibits some T- and B-cell function.
- CVS effects
- Vasodilation (leading to warmth/flushing/headache)
- Oedema (due to increased capillary permeability)
- Hypotension, and reflex increase in chronotropy
- Histamine has direct inotropic effects on the heart too.
- Increased bronchial reactivity – bronchoconstriction, especially in asthmatics.
- Neurological system
- Increased nociception
- Increased itch.
- May be related to satiety and weight gain (explains the weight gain when patients are given long term anti-psychotics)
- Increased smooth muscle contraction (abdominal pain, diarrhea)
- Increased secretion of gastric acid
- Initiates the triple response.