T Lymphocytes

William R Heath, The Walter and Eliza Hall Institute of Medical Research, Post Office Royal Melbourne Hospital, Melbourne, Victoria, Australia

To define the term 'T lymphocyte' it is first necessary to provide a definition for the word 'lymphocyte'. These cells were originally defined by morphological criteria to have a large nucleus to cytoplasm ratio, with cytoplasm rich in free ribosomes, containing a number of mitochondria, a small Golgi area and few lysosomes. Lymphocytes are found in high proportions in the bloodstream, lymphoid organs and the lymphatics. They have been divided into small, medium and large, although their size distribution is continuous rather than trimodal.

The term T lymphocytes (or T cells) was originally given to 'thymus-derived lymphocytes' by Roitt and colleagues (1969) several years after the discovery by Miller (1961) of a population of circulating lymphocytes that was produced in this organ. This definition was used to refer to those lymphocytes that matured in, and were exported from, the thymus rather than cells that simply occupied this organ. Host cells found in the thymus are referred to as 'thymocytes'. These consist of T lymphocyte progenitors at various stages of differentiation.

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