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Hypersensitivity is the term used when an immune response results in exaggerated or inappropriate reactions harmful to the host. Generally speaking, hypersensitivity reactions occur in response to external stimuli (antigens) whereas autoimmune reactions (see Chapter 66) occur in response to internal stimuli (antigens). The term allergy is often equated with hypersensitivity but more accurately should be limited to the IgE–mediated reactions discussed later in the section “Type I: Immediate (Anaphylactic) Hypersensitivity.”

The clinical manifestations of these reactions are typical in a given individual and occur on contact with the specific antigen to which the individual is hypersensitive. The first contact of the individual with the antigen sensitizes (i.e., induces the antibody), and subsequent contacts elicit the allergic response.

Hypersensitivity reactions can be subdivided into four main types. Types I, II, and III are antibody-mediated, whereas type IV is cell-mediated (Table 65–1). Type I reactions are mediated by IgE, whereas types II and III are mediated by IgG. The immunologic reactions are summarized in Table 65–1. The clinical manifestations of the hypersensitivity reactions are described in Table 65–2.

TABLE 65–1Immunologic Aspects of Hypersensitivity Reactions
TABLE 65–2Clinical Manifestations of Hypersensitivity Reactions

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