Three known lineages of antigen-presenting molecules restrict T-cell responses to microbial antigens: MHC class I and MHC encoded class I like molecules present peptides derived from the proteolysis of intracellular pathogens, MHC class ii molecules present peptides derived from the proteolysis of extracellular pathogens and CD1 molecules present unique microbial lipids and glycolipids. Recent studies have indicated that CD1 molecules mediate a novel system of antigen presentation and that MHC-encoded class I-like molecules can present unique subsets of intracellularly derived peptides.