Specific Immune Recognition by Lymphocytes: An Evolutionary Perspective

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Abstract
The origins of specific immune recognition by lymphocytes were discussed. The phenomena of immunity in invertebrates and the cells that might be involved in these processes were considered. The existence of vertebrate-type immunocompetent lymphocytes in invertebrates is not yet proven. All vertebrates apparently possess immunologically competent lymphocytes. Specific antigen recognition by lymphocytes of all vertebrates appears to be mediated by membrane-bound immunoglobulins [Ig]. These Ig are probably distinct from secreted, serum Ig (antibodies), although serum and surface Ig share combining sites for antigen which are formed of variable regions. Evidence for similar functional divisions among the immune systems of all vertebrates is reflected in the results of anatomical, functional, and physicochemical investigations of lymphocytes from animals as diverse as fish and mammals.