Mechanisms of antigen presentation

T-lymphocytes recognize short peptide antigens bound stably to polymorphic major histocompatibility complex (MHC)-encoded glycoproteins expressed on t he surface of antigen-presenting cells (APC). Two general pathways have evo lved to generate peptide-MHC complexes. The MHC class II antigen processing pathway provides a mechanism for sampling proteins present in endosomal co mpartments. CD4+ regulatory T-cells recognize peptides bound to MHC class I I molecules, which are selectively expressed in specialized APC that have e fficient mechanisms for uptake of microbial antigens, and express costimula tory molecules required for activating naive T-cells. CD8+ T-cells recogniz e peptides bound to MHC class I molecules. Class I molecules are widely exp ressed and bind peptides derived from the normal turnover of cellular prote ins, providing a mechanism to display a sampling of cellular components to be monitored for abnormalities by cytotoxic T-cells. Specialized accessory proteins influence the efficiency of antigen presentation and the specifici ty of immune responses through their roles in generating peptides, targetin g antigen and MHC glycoproteins to selected intracellular compartments, and by direct participation in the peptide-loading mechanism. It has recently been discovered that some viruses have evolved ways to inhibit or subvert d iscrete steps in antigen processing, providing a mechanism to evade immune recognition.