the health impacts of these allergens; and (3) to determine the adequacy of the knowledge base that is currently available on this topic.

This Executive Summary presents an overview of the committee's findings, followed by a section that presents all of the committee's recommendations and research agenda items from throughout the report.

The Allergic Reaction

An allergen is a biological or chemical substance that causes an allergic reaction. An allergy, in turn, is the immune-mediated state of hypersensitivity that results from exposure to an allergen. The symptoms commonly associated with allergy and allergic reactions are conjunctivitis (red, irritated eyes), rhinitis (a stuffy, runny nose, or ''hay fever"), and bronchitis (cough and congestion). Allergic reactions are classified and described in many ways, including: on the basis of the allergenic substance that causes them (such as cat dander, or pollen); according to the specific immunologic reactants (such as the types of antibodies or immune system cells involved in the reaction); or on the basis of the resulting disease (such as allergic asthma or hypersensitivity pneumonitis).

The most common allergic reaction is mediated by the immunoglobulin E (IgE) antibodies of the immune system. Mast cells, which contain histamine and other chemicals involved in the inflammatory response, also play a key role. After an initial exposure to a particular allergen, an individual who develops an allergy becomes "sensitized" to that allergen. In biological terms, sensitization occurs when IgE antibody specific for that allergen attaches to the surface of the individual's mast cells—making the individual "sensitive" (or "hypersensitive") to additional exposures. Subsequent exposure of the sensitized individual to the same allergen causes the mast cells to respond by releasing histamine and other inflammatory-response agents. These agents, in turn, interact with the surrounding tissues, causing an allergic reaction and the symptoms that we commonly associate with allergies (see Figure 2).

Everyone produces some IgE, but people with genetic predispositions to allergy (a condition called atopy) produce significantly greater quantities of it. In addition, the production of IgE antibody can continue for years after an encounter with an allergen. Thus, for example, someone who had an allergic reaction to penicillin as a child could still be allergic to the drug as an adult.

The reason why some people develop allergies and others do not is unclear. We do know, however, that genetic factors have a major influence on whether an individual develops allergy-related medical problems. For example, if one parent has allergies and the other does not, the chances are one in three that each of their children will have allergies. If both parents

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