More than 50 million Americans suffer from allergic diseases. One of the most common forms of allergies is an allergic reaction to something in the air. Health experts estimate that 35 million Americans suffer from upper respiratory tract symptoms that are allergic reactions to airborne allergens (substances that can cause an allergic reaction).

 

This article discusses airborne allergies. Additional eMedTV articles focus on other types of allergies, such as allergies to latex or certain foods. Click Food Allergy and Latex Allergy for more information on these topics.

 

Definition of an Allergy

Allergies occur when the body's immune system reacts to a normally harmless substance -- one that does not bother most people. People who have allergies are often sensitive to more than one substance. Examples of allergens that cause allergic reactions 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 a person with allergies 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 an antibody called immunoglobulin E, or IgE. Each IgE antibody is specific for one particular substance. In the case of pollen allergies, 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.

 

The IgE molecules are special because they are the only type of antibody that attaches tightly to the body's mast cells (a certain type of tissue cell) and to basophils (a type of blood cell). The next time the allergen encounters its specific IgE, it attaches to the antibody like a key fitting into a lock. This action signals the cell connected to the IgE to release (and in some cases, to produce) powerful chemicals that cause inflammation. Histamine is one of these chemicals. These chemicals act on tissues in various parts of the body, such as the respiratory system, and cause allergy symptoms.