Activation of Mast Cells

Mast cells are activated by cross-linking of FceRI molecules, which occurs by binding of multivalent antigens to the IgE molecules that are attached to the Fc receptors (Fig. 19-4). In an individual allergic to a particular antigen, a large proportion of the IgE bound to FceRI on the surface of mast cells is specific for that antigen. Exposure to the antigen will cross-link sufficient IgE molecules to trigger mast cell activation. In contrast, in nonatopic individuals, the IgE molecules bound to mast cells are specific for many different antigens, all of which may have induced low levels of IgE production. Therefore, no single antigen will cross-link enough of the IgE molecules to cause mast cell activation.

Immediate hypersensitivity reactions can be mimicked by injection of anti-IgE antibody instead of antigen.

IgE-coated resting mast cell

Antigen-activated mast cell

Antigen-

cross-linked

IgE/FceRI

Histamine/lipid Cytokines mediators

Antigen-

cross-linked

IgE/FceRI

Histamine/lipid Cytokines mediators t

Vascular/

Inflammation:

smooth

late phase

muscle

reaction

response:

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