MHC molecules, or Major Histocompatibility Complex molecules, are proteins found on the surface of cells that play a critical role in the adaptive immune response by presenting peptide fragments derived from proteins to T cells. They are essential for the recognition of self and non-self by the immune system, enabling T cells to distinguish between healthy body cells and potentially harmful pathogens. MHC molecules come in two main classes, Class I and Class II, each with distinct functions in immune surveillance.