MHC class I molecules are expressed on the surface of all nucleated cells in an organism. They have an essential role in the presentation of “self” intracellular antigens. When cells become infected by intracellular pathogens, such as viruses, MHC class I proteins bind and display protein fragments of the target pathogen on the cell surface to indicate to other components of the immune system that the cell needs to be destroyed.
MHC class II molecules are only expressed by a subset of immune cells called antigen-presenting cells. MHC class II recognition is essential for initiation of the adaptive immune response, through signaling to lymphocytes such as the CD4+ helper T cell.