Antigens that elicit immediate hypersensitivity reactions (allergens) are proteins or chemicals bound to proteins to which the atopic individual is chronically exposed. Typical allergens include proteins in pollen, house dust mites, animal dander, foods, and chemicals like the antibiotic penicillin. It is not known why some antigens induce strong TH2 responses and allergic reactions whereas others do not. Two important characteristics of allergens are that individuals are exposed to them repeatedly and, unlike microbes, they do not generally stimulate the innate immune responses that are associated with macrophage and dendritic cell secretion of TH1- and TH17-inducing cytokines. Chronic or repeated T cell activation in the absence of strong innate immunity may drive CD4+ T cells toward the TH2 pathway, as the T cells themselves make IL-4, the major TH2-inducing cytokine (see Chapter 9).
The property of being allergenic may also reside in the chemical nature of the antigen itself. Although no structural characteristics of proteins can definitively predict whether they will be allergenic, some features are typical of many common allergens. These features include low to medium molecular weight (5 to 70 kD), stability, gly-cosylation, and high solubility in body fluids. Anaphylac-tic responses to foods are typically induced by highly glycosylated small proteins. These structural features probably protect the antigens from denaturation and degradation in the gastrointestinal tract and allow them to be absorbed intact. Curiously, many allergens, such as the cysteine protease of the house dust mite Dermatophagoides pteronyssinus and phospholipase A2 in bee venom, are enzymes, but the importance of the enzymatic activity in triggering immediate hypersensitivity reactions is not known.
Because immediate hypersensitivity reactions are dependent on CD4+ T cells, T cell-independent antigens such as polysaccharides cannot elicit these reactions unless they become attached to proteins. Some drugs, such as penicillin, often do elicit strong IgE responses. These drugs react chemically with amino acid residues in self proteins to form hapten-carrier conjugates, which stimulate TH2 responses and IgE production.
The natural history of antigen exposure is an important determinant of the amount of specific IgE antibodies produced. Repeat exposure to a particular antigen is necessary for development of an allergic reaction to that antigen because switching to the IgE isotype and sensiti-zation of mast cells with IgE must happen before a hyper-sensitivity reaction to an antigen can occur. Individuals with allergic rhinitis or asthma often benefit from a geographic change of residence with a change in indigenous plant pollens, although environmental antigens in the new residence may trigger an eventual return of the symptoms. A dramatic example of the importance of repeated exposure to antigen in allergic disease is seen in cases of bee stings. The proteins in the insect venoms are not usually of concern on the first encounter because an atopic individual has no preexisting specific IgE antibodies. However, an IgE response may occur after a single encounter with antigen, and a second sting by an insect of the same species may induce fatal anaphylaxis! Similarly, exposures to small amounts of peanuts can trigger fatal reactions in previously sensitized individuals.
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