The role played by major histocompatibility complex proteins (MHC-1 and MHC-2) is to -

1. Major Histocompatibility Complex: MHC molecules or human leukocyte antigens (HLA) serve as a unique identification marker for every individual as the genetic sequence of MHC genes is different for every individual. The genes on chromosome 6TH (6p21) are clustered in three regions named as MHC region- I, II and III MHC class I presents endogenous antigens, while MHC class II presents exogenous antigens. 2. Biologic Impoance of MHC: It is now known that MHC molecules bind to peptide antigens and present them to T cells. Thus, these transplantation antigens are responsible for antigen recognition by the TCR(T Cell receptors ). Antibody molecules interact with antigen directly; the TCR only recognizes antigen presented by MHC molecules on the APC.(antigen prrsenting cells) The TCR is specific for antigen, but the antigen must be presented on a self-MHC molecule. The TCR is also specific for the MHC molecule. If the antigen is presented by another allelic form of the MHC molecule in vitro (usually in an experimental situation), there is no recognition by the TCR. This phenomenon is known as MHC restriction. Peptide antigens associated with class I MHC molecules are recognized by CD8+ cytotoxic T lymphocytes, whereas class II-associated peptide antigens are recognized by CD4+ helper T cells. NOTE-immunoglobulin class-switching is determned by : constant regeion of H chain (CH) immunoglobulin class-switching is mediated by : cytokines released from T cells, IL-4, IL-5, IFN-g, and transforming growth factor-beta (TGF-b)