MHC molecules, or Major Histocompatibility Complex molecules, are proteins found on the surface of cells that play a crucial role in the immune system. They help the body recognize foreign substances, such as viruses and bacteria, by presenting pieces of these invaders, known as antigens, to T cells. This interaction is essential for initiating an 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 primarily found on specialized immune cells, like B cells and macrophages, and present antigens to CD4+ T cells.