The major histocompatibility complex (MHC) is a group of genes that play a crucial role in the immune system. MHC molecules are found on the surface of cells and help the body recognize foreign substances, such as viruses and bacteria. They present pieces of these substances, called antigens, to immune cells, enabling the immune system to respond effectively. 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 primarily found on specialized immune cells, like B cells and macrophages, presenting antigens to CD4+ T cells.