The adaptive immune system contains two major arms. The cellular arm leads to the production of CTLs, also called killer T cells. The humoral arm leads to the production of antibodies that are secreted by B cells. T-helper cells are important players in generating these responses.
Both cellular immunity and humoral immunity require the activation of a class of lymphocytes called T cells (T from thymus, where these cells mature). T cells recognize peptide antigens 8-20 amino acids in length that are presented to them by cell surface proteins encoded in the major histocompatibility complex (MHC) locus (in humans, the MHC is called HLA, from human /ymphocyte antigen). The two types of MHC molecules that present antigenic peptides are called class I and class II. Both class I and class II MHC molecules are integral membrane proteins that are composed of two polypeptide chains.
The MHC class I molecule is a heterodimer composed of a heavy chain of about 350 amino acids, which is encoded within the MHC locus, and a light chain of about 100 amino acids, ft microglobulin, which is encoded elsewhere. The structure of an MHC class I molecule is shown schematically in Fig. 8.2A, and as determined by X-ray crystallography in Fig. 8.2B. The MHC class I heavy chain consists of three extracellular domains called au a2, and a3, a transmembrane domain, and a cytoplasmic domain. ft microglobulin forms a fourth extracellular domain and is held in the complex by noncovalent interactions. The a1 and a2 domains, which are structurally related to one another, form a platform with helical walls. The walls form a groove in which the antigenic peptide, consisting usually of 8-10 amino acids, is anchored. The a3 domain and ft microglob-ulin are homologous (derived from a common ancestral polypeptide by gene duplication) and are members of the immunoglobulin (Ig) superfamily. They share sequence identity and have common structural features.
Immune System Players
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