Lya

,B cell Th2 cell.

Mediators

Cytokines t

Immediate hypersensitivity reaction (minutes after repeat exposure to allergen)

Late phase reaction (2-4 hours after repeat exposure to allergen)

FIGURE 19-1 Sequence of events in immediate hypersensitivity reactions. Immediate hypersensitivity diseases are initiated by the introduction of an allergen, which stimulates Th2 reactions and IgE production. IgE sensitizes mast cells by binding to FceRI, and subsequent exposure to the allergen activates the mast cells to secrete the mediators that are responsible for the pathologic reactions of immediate hypersensitivity.

antigen exposure or IgE production. Such reactions are said to be nonatopic. • Immediate hypersensitivity reactions are manifested in different ways, depending on the tissues affected, including rashes, sinus congestion, bronchial constriction, abdominal pain, diarrhea, and systemic shock. In the most extreme systemic form, called anaphylaxis, mast cell-derived mediators can restrict airways to the point of asphyxiation and produce cardiovascular collapse leading to death. (The term anaphylaxis was coined to indicate that antibodies, especially IgE antibodies, could confer the opposite of protection [prophylaxis] on an unfortunate individual.) We will return to the pathogenesis of these reactions later in the chapter.

With this introduction, we proceed to a description of the steps in the development and reactions of immediate hypersensitivity.

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