• local acting substance
• has wide range of effects: vascular (dilates small blood vessels and increase capillary permeability); respiratory (bronchial constriction); gastrointestinal (increased secretion of acid; CNS (neurotransmitter)
• present in almost all body tissues
• synthesised and stored on mast cells and basophils – release from these cells is stimulated by allergic and non-allergic mechanisms:
• allergic – increased production of IgE antibodies in response to specific antigens  bind to surface of mast cells and basophils – on re-exposure to antigen, it is bound to antibody  mobilisation of intracellular calcium  histamine storage granules fuse with cell membrane  release contents
• non-allergic – some drugs act directly on mast cells  release histamine; cell injury also releases histamine
• two types of histamine receptor:
• H1 – involved in vasodilation, increase capillary permeability, bronchoconstriction, itching and pain from direct stimulation of sensory nerves, sedation (from action as a neurotransmitter)
• H2 – involved in increase secretion of gastric acid
• allergic reactions are mediated by histamine

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