The article discusses the pathways of antigen processing and presentation by MHC-I and MHC-II molecules in immune cells. It highlights the mechanisms by which cells acquire and degrade antigens, and how these are presented to T cells for immune recognition. Antigen processing involves the conversion of antigens into peptides that can bind to MHC molecules. For MHC-I, peptides are generated by proteolysis in the cytosol and translocated into the ER by the TAP complex, where they bind to MHC-I molecules. For MHC-II, peptides are generated by lysosomal proteolysis and presented in endocytic and phagocytic pathways. The article also describes the role of various proteins, such as tapasin, ERp57, and the invariant chain, in the processing and presentation of antigens. It emphasizes the importance of proteolytic systems, including the proteasome and lysosomal proteases, in generating peptides for MHC binding. The article also discusses the concept of cross-presentation, where antigens from exogenous sources are presented by MHC-I to CD8+ T cells. The role of various factors, such as the ER aminopeptidase (ERAAP) and the invariant chain, in shaping the peptide repertoire presented by MHC molecules is also discussed. The article concludes by highlighting the importance of epitope selection and the influence of various factors on the presentation of antigens by MHC molecules.The article discusses the pathways of antigen processing and presentation by MHC-I and MHC-II molecules in immune cells. It highlights the mechanisms by which cells acquire and degrade antigens, and how these are presented to T cells for immune recognition. Antigen processing involves the conversion of antigens into peptides that can bind to MHC molecules. For MHC-I, peptides are generated by proteolysis in the cytosol and translocated into the ER by the TAP complex, where they bind to MHC-I molecules. For MHC-II, peptides are generated by lysosomal proteolysis and presented in endocytic and phagocytic pathways. The article also describes the role of various proteins, such as tapasin, ERp57, and the invariant chain, in the processing and presentation of antigens. It emphasizes the importance of proteolytic systems, including the proteasome and lysosomal proteases, in generating peptides for MHC binding. The article also discusses the concept of cross-presentation, where antigens from exogenous sources are presented by MHC-I to CD8+ T cells. The role of various factors, such as the ER aminopeptidase (ERAAP) and the invariant chain, in shaping the peptide repertoire presented by MHC molecules is also discussed. The article concludes by highlighting the importance of epitope selection and the influence of various factors on the presentation of antigens by MHC molecules.