A histamine antagonist is an agent that serves to inhibit the release or action of histamine. Antihistamine can be used to describe any histamine antagonist, but it's usually reserved for the classical antihistamines that act upon the H1 histamine receptor. Antihistamines are used as treatment for allergies. Allergies are caused by an excessive response of the body to allergens, such as the pollen released by grasses and trees. An allergic reaction indicates an excessive release, by the body, of… (More on Anti-histamine)