1. Biological amine
  1. 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+.
  2. 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
  3. 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.
    • Respiratory
      • 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)
    • GIT
      • Increased smooth muscle contraction (abdominal pain, diarrhea)
      • Increased secretion of gastric acid
    • Skin
      • Initiates the triple response.
  1. Location of histamine
    • Most histamine is sequestered and bound in granules in mast cells and basophils.
    • Non mast cell histamine may be found in the brain, where it acts as a neurotransmitter. It is also found in the enterochromaffin-like cell (ECL cells) in gastric mucosa.
  2. Release of histamine
    • Mast cells and basophils release histamine of IgE binds to their surface.
    • Complement may also bind to mast cells, releasing histamine.
    • It is possible for drugs (e.g. morphine) to release histamine.
    • Direct mechanical injury also releases histamine.