A new foreign policy: MHC class I molecules monitor the outside world.

Although most cells exclusively use their major histocompatibility complex (MHC) class I molecules to present peptides from endogenous proteins, phagocytes also use them to present exogenous antigens. Here, Kenneth Rock describes how this novel antigen-presenting pathway may play an important role in immune surveillance for intracellular bacteria or parasites, as well as for viral infections and tumors affecting somatic tissues.