Degeneracy, mimicry and crossreactivity in immune recognition.

Degeneracy of recognition of antigen by the immune system is being used as an argument that the self-nonself discrimination cannot be a property of the specificity of its antigen-receptors, TCR and BCR, but must rely on emergent properties derived from a set of complex interactions and pathways. This essay analyzes an alternative view by showing that degeneracy and specificity are not mutually exclusive properties. The self-nonself discrimination is the sole evolutionary selection pressure for the specificity of the TCR and BCR, which can be quantitated as a "Specificity Index." Degeneracy is a non-issue for the self-nonself discrimination largely because it is a problem of chemistry, not of biology.