Some problems with this definition

As with all definitions, it can be somewhat problematic when taken too literally. Lymphocytes can be broadly divided into three groups: the B lymphocytes, the T lymphocytes and natural killer (NK) cells. NK cells also have receptors for antigens, but the antigens these receptors recognize appear to be the MHC molecules themselves. In contrast to T lymphocytes, these cells are activated when their receptors fail to encounter such molecules on target cells. Thus, NK cells kill cells that do not express MHC molecules. NK cells are not classically encompassed by the definition of the T lymphocyte, but they can be generated in the thymus, thus fulfilling the definition of a thymus-derived lymphocyte. However, they do not express the CD3 complex, which is expressed by 78 and afj T lymphocytes. This criterion becomes more relevant when we consider that small numbers of lymphocytes with indistinguishable properties to those of T lymphocytes can be generated extrathymically. For example, in nude mice, which do not possess a thymus, there are some 1 lymphocytes present in the circulation.

Thus, while the classical definition of the T lymphocyte is a 'lymphocyte that is derived from the thymus', such cells may be more accurately described as 'lymphocytes that express the T cell receptor and CD3 complex and are generally derived from the thymus'.

See also: B lymphocytes; CD3; CD4; CD8; H2 class I; H2 class II; Lymphocytes; T cell receptor, «0; T cell receptor, 78; T cell receptor, recognition by; Thy-1 ; T lymphocyte differentiation.

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