T cell receptors (TCRs)

are special proteins on the surface of T cells.  The T cells use them to recognise cells that are foreign, for example, a cell infected by a virus or a cancerous cell. Each T cell receptor recognises a specific fragment (peptide) of a protein displayed as a flag on cell-surface “flagpole” proteins called HLA molecules. So if a cell is infected with a virus or a cancer cell expresses proteins abnormally, “flags” of viral/abnormal proteins will be displayed to T cells and targeted by those T cells which have TCRs that recognise these peptide-HLA complexes. Within the body, many T cells express different receptors – this helps to ensure that there are always some T cells available to recognise and respond to any new threat that is encountered.