Specificity and degeneracy in antigen recognition: yin and yang in the immune system

One of the hallmarks of the immune system is specificity, a concept based on innumerable observations that antibodies react with the substance that elicited their production and only a few other structurally similar substances. The study of T cells has begun to suggest, however, that in responses mediated by their antibody-like receptors (T cell receptor or TCR) an individual T cell, expressing a singular TCR, can discriminate as exquisitely among antigens as the most specific antibodies but also exhibit "degeneracy": i.e., it can react with many disparate antigens (peptide-MHC complexes). An explanation for this duality (specificity and degeneracy) can be found in (i) the powerful amplifying signal transduction cascades that allow a T cell to respond to the stable engagement of very few TCR molecules, initially perhaps only one or two out of around 100,000 per cell, by their natural ligands (peptide-MHC complexes or epitopes on antigen-presenting cells--or APC) and (ii) the inverse relationship between TCR affinity for epitopes and epitope density (the number of copies of an epitope per APC). Older observations on the excess of total globulin production over specific antibody production in response to conventional immunization procedures suggest that B cells also exhibit degeneracy, as well as specificity. These views are developed against a backdrop describing how the author became interested in the immune system and has pursued that interest. "...a concept of science drawn from ...is [textbooks]...is no more likely to fit the enterprise that produced them than an image of a national culture drawn from a tourist brochure." Thomas Kuhn, Structure Of Scientific Revolutions