Major Histocompatibility Complex &Antigen Presentation:
MHC is a complex of chromosome loci that encode several proteins known as class I and class II MHC molecules. These molecules are unique for each individual. All nucleated cell have class I, while Antigen presenting cells contain class II.
During the processing and presentation of cytoplasmic antigen (Ag) for MHC I molecules (red pathwayin the figure below), cytoplasmic protein antigens are degraded by protease into 8 to 10 amino acid fragments that then enter the rough-surfaced endoplasmic reticulum (rER). In the rER, newly synthesized _ chains of MHC I molecules interact with both the processed antigen and _2 microglobulin (_2M) and form a stable complex. This complex leaves the rER via the typical secretory pathway through the Golgi apparatus. The antigen–MHC I complex is displayed on the cell surface, where it is available for recognition by cytotoxic CD8_ T lymphocytes.
MHC II molecules are assembled in the rER and then bind to an invariant chain, which blocks the antigen-binding site. At this point, the MHC II molecule and the invariant chain are secreted to the cell surface (blue pathwayin the figure below). After a brief stay on the cell surface, the MHC II molecule and invariant chain are endocytosed, and in an early endosome, the invariant chain is degraded. The foreign (exogenous) antigen is endocytosed and partially digested by proteolytic degradation in endosomes (white pathway in the figure below). The MHC II molecule can now bind the processed foreign antigen and return with it to the cell surface. On the cell surface, the antigen–MHC II complex is recognized by helper CD4_ T lymphocytes, which initiates the immune response. If the MHC II molecule fails to capture the antigen, it will be degraded in the lysosomal compartment (green pathway in the figure below).