Histamine is released from the mast cells and basophils when the body is exposed to foreign substances, or allergens, About.com explains. An instant allergic response begins as soon as the histamine is released by the body. While most allergic reactions are not harmful, they are not pleasant and medicines are often needed to relieve the symptoms. Antihistamine medications are used in treating these allergic reactions and are available in prescription form as well as over-the-counter.
Histamine forms when an amino acid called histidine undergoes decarboxylation, a process that eliminates a carboxylic acid from an organic compound. This reaction is started by an enzyme, L-histidine decarboxylase, which when deficient causes increased allergic reactions, News-Medical.net states. Histamine has several other functions that take place in the bowel and also acts as a chemical messenger, carrying signals within the nerves in the body. This chemical compound is found in all human beings as well as in most animals. First synthesized in 1907, histamine's pharmaceutical properties were shown in a 1911 demonstration, while the first antihistamine drugs were developed in 1943.