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The immune system is very specific and goal oriented. Although you may be allergic to a number of substances, allergic reactions are directed at specific allergens. For example, you may be allergic to Bermuda grass, but not oysters. At times, however, two or more foreign substances might appear similar in nature to the immune system, which may mistake one for the other and react to both. For example, if you are allergic to birch trees, your immune system may also react to apples or other fruits, which it mistakes for birch pollen. These cross-reactions occur because of similar allergens that are produced by a variety of plants. The allergic response, however, is by no means vague or ill-defined. It is a definite, vigorous attack aimed, unfortunately, at harmless agents. The end result is well-defined symptoms and disorders.

The deeper our understanding of the intricate nature of the allergic reaction, the more likely we are to find more effective treatments. We need to look more closely at the chain of events from the initial response to allergens to the many symptoms that may result. Although misguided, it is an efficient, well-orchestrated, and potentially explosive sequence of cellular and chemical interactions. This is the so-called "allergic cascade."

Who are the "players" in the allergic cascade?

Our body's immune system is designed to constantly be on the lookout for intruders. It has the remarkable ability to distinguish between "self" and "non-self" (foreign substances, which it tirelessly protects us from). Let us look more closely at this complex process. Take for example an exposure to ragweed pollen. Once in the body, the ragweed pollen is engulfed by the immune system's scouts, the so-called Antigen Preventing Cells or APC's. These APC's slice up the ragweed pollen into small fragments, which then combine with special proteins in the cell, called human leukocyte antigens or HLA's. HLA's function like a guideline to help the body distinguish "self" from "non-self." When combined with the HLAs, the fragments become visible to a key player in the allergic cascade (the lymphocytes), which recognizes them as foreign. This ragweed pollen fragment-HLA combination is exposed on the surface of the APC's in full view of these specialized white blood cells.

Before we review details of how the various players in the allergic cascade fulfill their roles, let's note these basic concepts of types of important cells and messenger proteins of the immune reaction:

The term white blood cells or leukocytes is derived from Greek words "leukos" meaning white and "cytes" meaning cells. The white blood cells are essential to the immune system and include the monocytes, macrophages, neutrophils, and lymphocytes.

Lymphocytes are white blood cells that play a key role in both immunity and allergy. They are divided into two types, the T and B lymphocytes. Each type is responsible for a particular branch of the immune system. It is the duty of the T-lymphocytes to be ready to directly shift into action to attack foreign substances (cell-mediated immunity). Some T-lymphocytes are experts at "killing" (cytotoxic or killer T cells) while others assist the immune response and are termed "helper" cells (TH cells). The TH cells are further divided into TH1 (infection fighters) and TH2 (allergy promoters), depending on the proteins they release. The partners of the T-lymphocytes are the B-lymphocytes. B-lymphocytes are tiny antibody factories that produce antibodies to help destroy foreign substances when stimulated to do so by the TH cells.

Basophils and eosinophils are other white blood cells that play an important role in allergy. T cells often call these cells into action in allergic conditions. Blood levels of eosinophils are commonly elevated in people with asthma and other allergic diseases.

Cytokines are a diverse group of proteins that are released by lymphocytes and macrophages in response to an injury or activation, such as by an allergen. They act as chemical signals that "step up" or "step down" the immune reaction.