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An allergy is a specific reaction of the body's immune system to a normally harmless substance, one that does not bother most people. Substances that cause allergies are called allergens. Types of allergens include:
Normally, the immune system functions as the body's defense against invading germs such as bacteria and viruses. In most allergic reactions, however, the immune system is responding to a false alarm. When an allergic person first comes into contact with an allergen, the immune system treats the allergen as an invader and gets ready to attack.
The immune system does this by generating large amounts of a type of antibody
called immunoglobulin E, or IgE. Each IgE antibody is specific for one
particular substance. In the case of pollen allergy, each antibody is specific
for one type of pollen. For example, the immune system may produce one type of
antibody to react against oak pollen and another against ragweed pollen.