Skip to Main Content


Hypersensitivity reactions are exaggerated or inappropriate immune responses to benign antigens. It is the immune response, not the antigens, that is harmful to the host. Usually, hypersensitivity reactions occur in response to external, or “non-self,” antigens (covered in this chapter), whereas autoimmune reactions (see Chapter 66) occur in response to internal, or “self,” antigens.

Hypersensitivity 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

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.