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To understand how antihistamines work, we must first know what Histamine is. Histamine is a protein released when an allergen (something innocuous the individual's immune system mistakes for a harmful substance) is detected by lymphocytes, setting off a complex chain of evens. Histamine then causes inflammation or contraction of smooth muscles. It can also increase inflammation by attracting more lymphocytes to the area, raising your immune system's level of attack on the particular allergen.

Antihistamines work by blocking histamine. One way they can do this is by calling off the lymphocytes that have been attracted to the "problem" by the Histamine release. They can also bind themselves to Histamine so it doesn't bind to other cells in your immune system. By binding itself to Histamine, antihistamines reduce the active amount in your system.

Antihistamines can be given topically, orally or intravenously. Some common antihistamines are Diphenhydramine (Benadryl), Loratadine(Claritin) and Fexofenadine hydrochloride (Allegra, Telfast).

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