Mast cells are present in the skin and mucosal epithelium and rapidly secrete proinflammatory cytokines and lipid mediators in response to infections and other stimuli.
We introduced mast cells in Chapter 2. Recall that these cells contain abundant cytoplasmic granules containing various inflammatory mediators that are released when the cells are activated, either by microbial products or by a special antibody-dependent mechanism. The granule contents include vasoactive amines (such as histamine) that cause vasodilation and increased capillary permeability, and proteolytic enzymes that can kill bacteria or inactivate microbial toxins. Mast cells also synthesize and secrete lipid mediators (such as prostaglandins) and cytokines (such as TNF). Because mast cells are usually located adjacent to blood vessels (see Fig. 2-1), their released granule contents rapidly induce changes in the blood vessels that promote acute inflammation. Mast cells express TLRs, and TLR ligands can induce mast cell degranulation. Mast cell-deficient mice are impaired in controlling bacterial infections, probably because of impaired innate immune responses. Mast cell products also provide defense against helminths and are responsible for symptoms of allergic diseases. We will return to a detailed discussion of mast cells in relation to allergic diseases in Chapter 19.
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