T lymphocytes mediate immune defenses against infectious diseases. The major histocompatibility complex class I and class II antigen-processing pathways play an essential role in the activation of pathogen-specific T lymphocytes by presenting peptide fragments derived from pathogen-encoded proteins. The past 5 years have seen remarkable advances in our understanding of both of these antigen-processing pathways. Many new proteins that are critical to T-cell-mediated defenses against infectious pathogens have been identified and characterized. Although the direct effects of aging on antigen processing and presentation are relatively unexplored, it is likely that immunosenescence affects and is affected by the antigen-processing pathways.