Figure 4. The T cell immune response.

T cells recognize antigen as small peptides derived from the breakdown of intracellular proteins displayed on the surface of antigen presenting cells in the context of major histocompatibility (MHC) antigens. Tumor antigens are cleaved by the proteasome into fragment peptides. The peptides transported by the transporter associated with antigen processing (TAP) into the endoplasmic reticulum, where they are loaded onto MHC molecules that are assembled and transported to the cell surface.

Activation of naïve T cells requires the interaction of an appropriate T cell receptor (TCR) and MHC-presented peptide antigen (Signal #1) and a co-stimulatory signal (Signal #2).

β₂M: beta-2 microglobulin

Adapted from: Kong HT, Restifo NP: Natural selection of tumor variants in the generation of “tumor escape” phenotypes. Nature Immunol 2002;3:999-1005.¹²