Antibodies can recognize antigen without the involvement of other molecules; the presence of antigen and antibody is sufficient for their interaction. In the course of their differentiation, B cells receive assistance from T cells by a process that will be described in some detail below. This process, literally called T-cell help, is antigen-specific, and the T cells responsible for providing it are helper T cells. Although antibodies contribute to the elimination of bacterial and viral pathogens, it may also be necessary to destroy infected host cells, which might serve as a source of new virus particles. This task is carried out by cytotoxic T cells. Both helper T cells and cytotoxic T cells make use of antigen-specific receptors encoded by genes that are generated by mechanisms analogous to those used by B cells to generate immunoglobulin genes—including gene rearrangements. However, T cells recognize their cognate antigens in a manner very different from that used by B cells. The antigen-specific receptors on T cells recognize short snippets of protein antigens, but can do so only when the snippets are part of a glycoprotein complex present on the external surface of an “antigen-presenting” cell. The genes that encode the membrane glycoprotein complex that presents the antigen snippets are present in a region of genomic DNA called the major histocompatibility complex (MHC). Various antigen-presenting cells, in the course of their normal activity, digest pathogen-derived proteins (as well as their own proteins) and then “present” physical complexes consisting of an MHC protein bound to a protein snippet (peptide) on their cell surface. T cells can scrutinize these complexes, and if they detect a pathogen-derived peptide bound to the MHC molecule, the T cells take appropriate action, which may include killing the cell that carries the MHC-peptide complex. In this section, we describe the MHC and the proteins it encodes, then examine how MHC molecules are involved in antigen presentation and antigen recognition by T cells.