The Major Histocompatibility Complex (MHC) is a group of genes that play a crucial role in the immune system. It helps the body recognize foreign substances, such as viruses and bacteria, by presenting pieces of these invaders, called antigens, on the surface of cells. This process enables T cells, a type of immune cell, to identify and attack infected or abnormal cells. MHC molecules are divided into two main classes: 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, like B cells and macrophages, and present antigens to CD4+ T cells. This interaction is essential for initiating and regulating immune responses.