Antihistamine / Histamine Antagonist.

Anti-histamines are a group of medications that block the effects of histamine, a chemical released in the body during an allergic reaction. These medications can prevent or reduce the symptoms of an allergic reaction.

An allergy is a condition in which the body becomes unusually sensitive to some substance, such as pollen, mold spores, dust particles, certain foods, or medicines. These substances, known as Allergens, cause no unusual reactions in most people. But in people who are sensitive to them, exposure to Allergens causes the immune system to overreact. The main reaction is the release of a chemical called Histamine from specialized cells in the body tissues. Histamine causes such familiar and annoying allergy symptoms as sneezing, itching, runny nose, and watery eyes.

As their name suggests, Anti-histamines block the effects of Histamine, reducing allergy symptoms. When used for this purpose, they work best when taken before symptoms are too severe. Anti-histamine creams and ointments may be used to temporarily relieve itching. Some Anti-histamines are also used to treat motion sickness, nausea, dizziness, and vomiting. And because some cause drowsiness, they may be used as sleep aids.

H1 antihistamines are used to relieve allergies, alleviating symptoms such as runny nose, itching, swelling, or asthma. H2 antihistamines suppress acid production by the stomach, and are used in the treatment of peptic ulcers, often making surgery unnecessary.

Some Anti-histamine products are available only with a physician's prescription. Others can be bought without a prescription. These drugs come in many forms, including tablets, capsules, liquids, injections and suppositories. Some common Anti-histamines are Astemizole (Hismanal), Brompheniramine (Dimetane, Dimetapp), Chlorpheniramine (Deconamine), Clemastine (Tavist), Diphenhydramine (Benadryl), Doxylamine (an ingredient in sleep aids such as Unisom and Vicks NyQuil), Loratadine (Claritin), and Promethazine (Phenergan).

References :
1.http://www.pharmaceutical-drug-manufacturers.com/pharmaceutical-drugs/anti-histamine.html
2.http://encyclopedia.farlex.com/Anti-histamine
3.http://allergies.about.com/od/glossaryofallergyterm1/g/antihistamine.htm