MHC, or Major Histocompatibility Complex, is a group of genes that play a crucial role in the immune system. These genes help the body recognize foreign substances, such as viruses and bacteria, by presenting pieces of these invaders on the surface of cells. This process is essential for the activation of T cells, which are vital for immune responses. 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 antigen-presenting cells and present to CD4+ T cells. This interaction is key for initiating an effective immune response.