The Major Histocompatibility Complex (MHC) is a group of genes that play a crucial role in the immune system. These genes produce proteins that help the body recognize foreign substances, such as viruses and bacteria. MHC molecules present pieces of these foreign substances, called antigens, on the surface of cells, allowing immune cells to identify and respond to potential threats. There are two main classes of MHC molecules: MHC class I and MHC class II. MHC class I molecules are found on almost all nucleated cells and present antigens to CD8+ T cells, while MHC class II molecules are primarily found on specialized immune cells, such as B cells and macrophages, presenting antigens to CD4+ T cells. This interaction is essential for initiating an effective immune response.