Thymus-dependent and thymus-independent populations: origin, migratory patterns and lifespan.

It is now accepted that there are present within the peripheral lymphoid tissues of the mammal, lymphocytes which differ in their origin, migratory patterns and lifespan; two major groups have so far been defined, one of thymus-dependent or thymus-derived lymphocytes and another of thymus-independent or bone-marrow derived lymphocytes. These cells which are morphologically indistinguishable when free floating in the blood or lymph nevertheless segregate from one another into clearly defined compartments whilst resident in or passing through the lymph nodes, spleen or Peyer's patches. The purpose of this article is to summarize some of the experimental evidence for this segregation and to discuss its physiological and immunological implications. It is not intended to be an extensive review of the literature.