Histamine is a nitrogenous organic compound produced by the action of histidine in the presence of decarboxylase. Many tissues, especially the mast cells of the skin, lung and intestinal mucosa, contain large amounts of histamine, a substance that can be released when tissues are damaged or when inflammatory or allergic reactions occur. Histamine has a strong diastolic effect and increases the permeability of the walls of capillaries and microvenules, allowing plasma to enter the tissues, resulting in a state of local edema. Histamine was once used in the examination of gastric secretory function to identify patients with gastric cancer and pernicious anemia for the occurrence of true gastric acid deficiency, but it is now rarely used. In addition, histamine desensitization, i.e., repeated intramuscular injections using small doses of histamine, is also used clinically to improve the patient’s tolerance to histamine and has some therapeutic effect.