How Antihistamines Work for Allergies

In this animation, it shows how the body produces White Blood Cells after the first exposure to an allergen. This happens to prepare the immune system for the next encounter with the same allergen. In the animation, it shows how antibodies attach to the Mast cells which are special cells found in the tissue of the respiratory and digestive system. After the body has been exposed again to the same allergen, this triggers the Mast cells to release Histamine, which attaches to the receptors near by cells. The receptors then interact with other substance in body and cause nearby blood vessels to swell and secrete the fluid, causing running nose, sneezing and watery eyes. Anti-histamines are drugs that basically attach themselves to the receptors and they block the histamine from binding to the cell receptors, thus preventing those symptoms.

http://www.youtube.com/watch?v=ywdk3BTjK2s&feature=related