Allergen

An allergen is a molecule that stimulates an allergic response in a sensitive individual. Allergens are almost always proteins; however, not all proteins are allergens. A protein that acts as an allergen displays two fundamental properties: induction of an IgE immune response on first exposure and induction of a clinical response to the same or similar protein on subsequent exposures. Some examples of allergens include dust mites, peanuts, pollen, and pet dander; however, a comprehensive list of allergens is impossible to assemble, as the list of possible substances is extensive (i.e., food, air particles, drugs, animal products, and insect stings). Sensitivities vary greatly from one person to another; and the response to allergens also varies in severity from allergic rhinitis and hives to asthma and anaphylaxis. Allergen sensitivity testing is used in epidemiology studies to characterize atopy and its association with allergic diseases in populations.

An allergen provokes an allergic response in the immune system by acting as a substance that stimulates an IgE-mediated antibody response. On first exposure to the allergen, IgE antibodies are developed that specifically recognize the allergen. These antibodies are then bound to mast cells, a cell type that is very common in our gastrointestinal and respiratory systems. If the body is exposed to the allergen again, the mast cell-bound IgE antibody recognizes the antigen and causes the mast cells to release substances (e.g., histamine, leukotrienes, and interleukins) that cause cell damage and inflammation, resulting in the symptoms we commonly associate with allergies such as a runny nose, congestion, hives, and swelling. Atopy refers to the condition of [Page 20]raised IgE antibody levels following exposure to common allergens.

Exposure to an allergen can occur through inhalation (e.g., pollen), ingestion (e.g., peanuts), or direct contact (e.g., latex gloves). If the allergen is in the air, the allergic reaction will likely occur in the eyes, nose, and lungs. If the allergen is ingested, the allergic reaction will likely occur in the mouth, stomach, and intestines. An allergic reaction can occur throughout the body, such as hives or analphlaxis, if the reaction is severe enough. Many protein allergens have been sequenced, but there are only a handful of common characteristics associated with these allergenic proteins. Several allergen types (food and plant) belong to only a few of the thousands of protein families, suggesting common structural features. Other common features include a high dose of the protein in the ingested or inhaled substance and resistance to digestion, two features that make immunologic resistance more difficult.

The goal of diagnosing an allergy is identifying the causative allergen; IgE antibodies specific to a particular allergen are measured in the blood in RAST (radioallergosorbent) testing, or skin-prick testing (SPT) is performed. SPT involves the controlled application of a variety of allergens and positive and negative controls to the skin. Itchy, red skin indicates a positive response to the applied allergen—that is, an IgE-mediated reaction. SPT has been used in a variety of ways in epidemiologic studies to characterize atopy in populations, including to identify the prevalence or incidence of reactivity to specific allergens in predisposed populations such as asthmatics; to identify a sensitive population for use in a caseseries or case-control study; to examine the trend of skin test reactivity over a particular time frame in a population; to calculate the association of specific skin reactivities with particular allergy types (e.g., asthma or chronic obstructive pulmonary disorder) or the severity of those allergies; and to calculate the association between reactivity to an allergen and the levels of that allergen in an environment (e.g., pollen reactivity and pollen counts).

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