Antibodies, mast cells, and eosinophils function with antibodies to mediate the expulsion and killing of some helminthic parasites. Helminths (worms) are too large to be engulfed by phagocytes, and their integuments are relatively resistant to the microbicidal products of neu-trophils and macrophages. They can, however, be killed by a toxic cationic protein, known as the major basic protein, present in the granules of eosinophils. IgE, IgG, and IgA antibodies that coat helminths can bind to Fc receptors on eosinophils and cause the degranulation of these cells, releasing the basic protein and other eosino-phil granule contents that kill the parasites. The high-affinity Fce receptor of eosinophils (FceRI) lacks the signaling P chain and can only signal relatively weakly through the associated y chain. In addition, IgE antibodies that recognize antigens on the surface of the helminths may initiate local mast cell degranulation through the high-affinity IgE receptor (see Chapter 19). Mast cell mediators may induce bronchoconstriction and increased local motility, contributing to the expulsion of worms from sites such as the airways and the lumen of the gastrointestinal tract. Chemokines and cytokines released by activated mast cells may attract eosinophils and cause their degranulation as well.
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