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38208_Ward's World+MGH Allergies

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2 Besides the intrinsic sensitizing properties of allergens, individual predisposition of the allergic person to become sensitized also plays an important role. Clinical manifestations, including hay fever, allergic asthma, and atopic dermatitis (Fig. 3), occur more frequently in some families; the inheritance of a capacity to develop these forms of allergy has been called atopy or the atopic state. In other clinical forms of allergy, genetic predisposition (although possibly present) is not as evident. Mechanisms Exposure to sensitizing allergens may induce several types of immune response, and the diversity of the associated im- munological mechanisms is responsible for the various clinical forms of allergic reactions that are encountered in practice. Three principal types of immune responses are encountered— the production of immunoglobulin E (IgE) antibodies, IgG or IgM antibodies, and sensitized lymphocytes. IgE antibodies IgE antibodies belong to a peculiar class of serum immuno- globulins. This class is responsible for the majority of allergies of the so-called immediate or anaphylactic type. IgE antibodies are present in blood in very small amounts (normally less than 0.2 micrograms per milliliter), but have the capacity to bind strongly to the membranes of tissue mast cells and blood ba- sophils (Fig. 1). These cells are the major effector components in an immediate allergic reaction. They contain a wide variety of preformed inflammatory mediators, such as histamine and serotonin. They also have the capacity to form, upon interac- tion with allergen, further substances that cause inflammatory changes in tissue (for example, increased vascular perme- ability that results in swelling and is manifested by redness and wheals). Among such mediators that are formed after cell stimulation, leukotrienes, the platelet-activating factor (PAF), and several substances that attract other blood cells (for example, eosinophils) to the site of reaction have gained the most attention. Mast cells and basophils may be compared to Allergy (continued) + ward ' s science Fig. 1: Aptenodytes patagonicus, the king penguin. (Credit: Gerald and Buff Corsi; copyright © California Academy of Sciences)

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