• adaptive immunity;
  • innate immunity;
  • T and B cells;
  • variable lymphocyte receptors


Adaptive immunity has been defined, principally through studies of avian and mammalian species, as the ability to mount specific immune responses to a virtually unlimited variety of antigens. A key feature of an adaptive immune system is the ability to remember previous encounters with antigens and to achieve a more rapid, heightened response on secondary encounter. Adaptive immune systems featuring an enormous anticipatory receptor diversity and specific memory have been defined only in vertebrates. Surprisingly, the adaptive immune systems in jawless and jawed vertebrates employ very different types of antigen receptors. This evolutionary inventiveness suggests that adaptive immunity provided additional fitness value over the previously existing innate immune mechanisms.