Histocompatibility antigens are proteins found on the surface of cells that play a crucial role in the immune system. These antigens help the body distinguish between its own cells and foreign cells, such as those from pathogens or transplanted organs. The most well-known histocompatibility antigens are the Major Histocompatibility Complex (MHC) molecules, which are essential for the immune response. There are two main classes of MHC molecules: MHC Class I and MHC Class II. MHC Class I molecules are present on almost all nucleated cells and present antigens to CD8+ T cells, while MHC Class II molecules are found mainly on immune cells like B cells and macrophages, presenting antigens to CD4+ T cells. This interaction is vital for initiating immune responses and maintaining self-tolerance.