Step 2: The role of the immune system

An allergic reaction occurs after your body becomes sensitized to an offending substance -- such as ragweed pollen, dust mites, or cat dander. All of these substances are called "allergens." In the four stages of an allergic reaction described below, ragweed pollen will be used as an example of how your immune system might react when an allergen gets into the body.

The body recognizes an intruder

The first time you breathe in ragweed pollen, your body identifies the pollen as a foreign, invading substance. Plasma cells, primarily located under mucus membranes, make large amounts of antibody against it. Your body can make a unique antibody for every type of pollen. In fact, it can make a unique antibody to every possible food ingredient, or anything else that comes from outside the body and is not "you." The type of antibody most commonly associated with allergic reactions is called Immunoglobulin E ("IgE"). The body has other ways to recognize and react to a substance; this example will focus on antibody reactions.

The body gets ready

The IgE antibodies to ragweed pollen attach themselves to mast cells (which are located in tissue that lines the nose, bronchial tubes, as well as the gastrointestinal tract and the skin) and basophils (cells which circulate around in the bloodstream). The antibodies stick out from the surface of the mast cells and basophils, waiting. Your body is now "primed" and ready for any time in the future when ragweed pollen enters the body.

The attack

The next time you breathe in ragweed pollen, the antibodies "grab" the pollen and send a signal down to the attached mast cells and basophils. Powerful chemicals are immediately released -- including histamine, heparin, and over 30 others. They cause the nearby tissue to become swollen and inflamed. The runny nose and sneezing are the body's way of trying to get rid of the pollen grains. The itching and other unpleasant effects may be the body's way of making you avoid contact with that allergen in the future.

The delayed response

Some people also will experience what is known as the "late phase" of an allergic reaction. This occurs because the chemicals associated with the reaction attract additional types of immune cells to the site of the allergic reaction, such as eosinophils, neutrophils, and lymphocytes. These cells, in turn, release chemicals which can actually cause tissue damage. The late phase can occur between 4 and 24 hours after exposure.

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