An allergy develops in much the same way as does an immunity against an infectious disease. Several things happen to cause an allergy. First, a person is exposed to a substance such as pollen. The bodies of most people simply ignore pollen, but others treat pollen as a foreign invader. When this happens, a substance in the blood known as an antigen becomes active.
Antigens cause the formation of antibodies whose job it is to fight off the invading substance.
Antibodies are normally in the blood and are activated by the antigen to which each is specifically linked. For instance, if a person who is allergic to shrimp eats the food, an antigen to shrimp will cause the formation of antibodies whose only function is to work against that one antigen. Antigens that cause allergies are called allergens. Heat, cold, sunlight and the emotions can also act as allergens. Of the two people in ten who have some sort of allergy, many belong to families in which parents and other close relatives also have allergies.