Table 1.

Definitions of terminology

Polyspecificity	This term emphasizes two important features of TCR recognition: the ability to recognize multiple distinct peptide/MHC ligands as well as the specificity with which each of these ligands is recognized. The workshop recommends general usage of this term.
Degeneracy	Emphasizes the finding that some of the peptide/MHC ligands recognized by a particular TCR can be distinct in both primary sequence and structure. In general, the term degeneracy is a better fit for peptide binding to MHC molecules than for TCR recognition. Peptides that are highly diverse in sequence can be bound by MHC molecules and even the requirements for anchor residues are typically not very strict. In contrast, subtle changes in a peptide can result in loss of TCR recognition.
Molecular mimicry	Polyspecificity in the context of autoimmune diseases: a selfreactive T cell is stimulated by microbial peptide(s) successfully processed from the microbe and bound to MHC molecules, leading to activation and expansion of autoreactive T cell populations. A large fraction of the early literature used this term.
Plasticity & Flexibility	One structural explanation for polyspecificity: in crystal structures of TCRs with and without peptide/MHC ligands, substantial movements in TCR loops were documented, in particular in the CDR3 loops. However, not all cases of polyspecificity may be caused by flexibility in TCR loops.
Crossreactivity	Similar to polyspecificity, but this term is not as explicit in emphasizing the existence of multiple peptide/MHC ligands. This term was originally used to indicate unexpected reactivity to targets that differed from those used to initially define the clone.