Explain how an allergic reaction occurs.

When a person who is allergic to a particular allergen comes in contact with it, an allergic reaction occurs. This reaction begins when the allergen (for example, pollen grains) enters the body and causes special cells in the body tissues to make a chemical called an antibody. When the allergen comes in contact with the antibodies, a substance called histamine is released. Histamine causes allergic reaction and results in swelling and inflammation.