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Hypersensitivity reactions are exaggerated or inappropriate immune responses to benign antigens. It is the immune response, not the antigens, which are harmful to the host. Usually, 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 immunoglobulin (Ig) E-mediated reactions discussed later in the section “Type I: Immediate (Anaphylactic) Hypersensitivity.” Note that many autoimmune diseases have pathologic features of one or more hypersensitivity reactions. This chapter will cover the patterns of hypersensitivity reactions using examples of diseases in which there are known external triggers. Chapter 66 will cover autoimmune diseases, in which the immune reactions are largely against are self-antigen(s).

These reactions are antigen-specific, meaning that the first contact with the antigen sensitizes the immune system (i.e., primes the adaptive immune system), and subsequent contacts elicit the hypersensitive (allergic) response. Within an individual, these subsequent antigen exposures elicit similar clinical manifestations, although the severity of the hypersensitivity reactions may increase with time.

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). 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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