A new concept of immune specificity emerges from a consideration of the self-nonself discrimination.

The necessity to make a Self(S)-NonSelf(NS) discrimination is the evolutionary selection pressure for specificity of the immune response. A new definition of paratopic specificity, which is heuristic and generalizable, can be derived from an understanding of this selection pressure. Specificity of the paratope is defined by a Specificity Constant, K, which is the probability that a functional change in recognition will be anti-Self. In an antigen-unselected population, K is the proportion of cells that are anti-Self. This definition is unique in that it is derived from the function upon which evolution selects, namely the effector output. This paper describes how the concept of a Specificity Constant was derived, how it is estimated, what it can be used to explain, and how it impacts on repertoire and effectiveness of response.