<prompt> The role played by major histocompatibility complex proteins MHC-1 and MHC-2 is to -Transduce the signals to T cells following antigen recognitionMediate immunoglobulin class-switchingPresent antigens for recognition by T cell antigen receptorsEnhance the secretion of cytokines  <response>: 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 TCRT 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 