Osmosis video - MHC class I and MHC class II molecules

MHC class I molecules are found on the surface of all nucleated cells in the body. These molecules present peptides from within the cell to the immune system, and play an important role in alerting the immune system about cells infected by viruses. On the other hand, MHC class II molecules are found only on antigen-presenting cells, such as macrophages, dendritic cells, and B cells. These molecules present peptides from extracellular pathogens to the immune system.