T cells recognize only fragments of proteins bound to histocompatibility proteins.
I antigens bind peptides derived from proteins synthesized by the cell which displays them, while peptides derived from proteins in the environment are taken up by endosomes, degraded by proteases in vesicles and then bound to class II proteins and displayed on the cell surface. Class I proteins are expressed by nearly all normal cells, while class II expression is constitutively limited to monocytes, dendritic cells, and B cells. Many other cells can be induced to express class II by g -interferon including endothelial cells. These cells are associated with lytic responses. These cells then produce cytokines.