The purpose of an antihistamine is to work against the effects of histamine, a chemical substance found in nearly all body tissues.
Histamine is released in response to injury by foreign substances such as pollens.
It is also the irritating agent in certain insect venoms. It appears to play a protective role in body chemistry, but its functions are not completely known. Histamines produce varied effects in the body, including widening of the blood vessels, constriction of the smooth muscles of the lungs, and stimulation of gastric secretion. Some antihistamines are taken to relieve allergies such as hay fever and allergic skin problems. Other antihistamines are used to treat motion sickness and vomiting.