NKT cells are not MHC restricted and do not recognize peptides displayed by antigen-presenting cells. These cells express ap TCRs that are CD1 restricted and also bear a surface marker found on NK cells, hence their name. The TCRs of NKT cells recognize lipid antigens presented by CD1 molecules. CD1 molecules are class I MHC-like molecules made up of a heavy chain and p2-microglobulin. The heavy chain has a groove made up of hydrophobic residues that can bind and present lipid antigens. These lipid antigens may be derived from endocytosed microbes or they may be self lipids (see Chapter 6). In the thymic cortex, double positive aP T cells that express T cell receptors that recognize CD1 molecules expressed on neighboring double positive thymocytes are induced to differentiate into NKT cells. A large number of CD1-restricted NKT cells have an "invariant" TCR resulting from a unique and stereotypic TCR a chain gene rearrangement event. NKT cells secrete cytokines and participate in host defense and may also serve to regulate a variety of immune responses. The functions of NKT cells are described in Chapter 10.
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