srjmsSRJMSSRJMS2788-9483https://doi.org/10.47310/srjms.2022.v02i01.003Histamine: Insights into Structure, Synthesis, Metabolism and the Physiological ActionsAtaSedik IbrahimElsayedAzabElsayedRabiaA. M.YahyaBackground: Histamine is a biogenic amine produced from different tissues and organs as skin, lung and digestive system. It play important role in regulation of sensation of pain, body temperature, memory, and wakefulness. Objectives: The current review aimed to highlight on the structure, metabolism and the physiological effects of histamine. Histamine is synthesized from the L‐histidine amino acid in both the neuronal cell bodies and axon terminals and stored in vesicles in axon varicosities. In the brain, histamine is also produced by mast cells. The release and synthesis of histamine are regulated by H3 receptors. The action of histamine is modulated by four types of G protein coupled receptors (H1, H2, H3, and H4). The histamine releasing and synthesizing neurons are found in tuberomamillary nucleus in hypothalamus. It is a key player in allergic reactions, circadian cycle regulation, vascular permeability, gastric and neurological functions, immune cell differentiation, epithelium proliferation, neoplastic progression and angiogenesis. Release of toxic substance from ischemic tissue as tissue enzyme, histamine, and serotine is the main cause of circulatory shock. In inflammatory responses and allergic reactions, histamine causes arterioles vasodilator and increases the porosity of capillaries to leak plasma and plasma proteins into interstitial space leads to edema. The rate of formation and secretion of hydrochloric acid by the parietal cells is directly related to the amount of histamine secreted by the enterochromaffin-like cells, that may be stimulated by hormones secreted by the enteric nervous system of the stomach wall. Histamine and slow reactive substance of anaphylaxis released in the lung tissues during allergic reactions by tissue mast cells by the action of allergic substances as pollen grains, irritant chemicals or vapors and cause bronchial constriction. Histamine intolerance results from a disequilibrium of accumulated histamine and the capacity for histamine degradation. Ingestion of histamine-rich food, alcohol, or drugs that release histamine or block diamine oxidase may provoke diarrhea, headache, congestion of the nose, asthmatoid wheezing, hypotension, arrhythmia, urticaria, pruritus, flushing, and other conditions in these patients. Conclusion: It can be concluded that the action of histamine is modulated by H1, H2, H3, and H4 receptors. It is a key player in allergic reactions, circadian cycle regulation, vascular permeability, gastric and neurological functions, immune cell differentiation, epithelium proliferation, neoplastic progression and angiogenesis. The rate of formation and secretion of hydrochloric acid by the parietal cells is directly related to the amount of histamine secreted by the enterochromaffin-like cells. The release of histamine may be provoke diarrhea, congestion of the nose, headache, asthmatoid wheezing, hypotension, arrhythmia, urticaria, pruritus, and flushing.