What is a T-cell receptor?

The T-cell receptor (TCR) is a protein complex found on the surface of T cells, or T lymphocytes,[1] that is responsible for recognizing fragments of antigen as peptides bound to major histocompatibility complex (MHC) molecules. The binding between TCR and antigen peptides is of relatively low affinity and is degenerate: that is, many TCRs recognize the same antigen peptide and many antigen peptides are recognized by the same TCR.

Explanation:

a- all cells come from pre-existing cells