FIGURE 19-3 Polypeptide chain structure of the high-affinity IgE Fc receptor (FceRI). IgE binds to the Ig-like domains of the a chain. The p chain and the y chains mediate signal transduction. The boxes in the cytoplasmic region of the p and y chains are ITAMs, similar to those found in the T cell receptor complex (see Fig. 7-5). Lyn and Syk are tyrosine kinases that bind to the p and y chains, and participate in signaling events. A model structure of FceRI is shown in Chapter 12.

the signaling subunit for FcyRI, FcyRIIIA, and FcaR and is called the FcR y chain (see Chapter 12). Tyrosine phosphorylation of the ITAMs of the P and y chains initiates the signaling cascade from the receptor that is required for mast cell activation, described next. The FceRI on eosinophils and several other cell types lacks the P chain, so signaling is mediated only by the y chains in these cells.

The importance of FceRI in IgE-mediated immediate hypersensitivity reactions has been demonstrated in FceRI a chain knockout mice. When these mice are given intravenous injections of IgE specific for a known antigen followed by that antigen, anaphylaxis does not develop or is mild, whereas it is a severe reaction in wild-type mice treated in the same way. FceRI expression on the surface of mast cells and basophils is increased by IgE, thereby providing a mechanism for the amplification of IgE-mediated reactions.

Another IgE receptor called FceRII, also known as CD23, is a protein related to C-type mammalian lectins whose affinity for IgE is much lower than that of FceRI. The biologic role of FceRII is not known.

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