Abstract
Abstract T lymphocytes (or T cells) are thymus‐derived and are key players in the adaptive cellular immunity. They recognize antigen by means of their T‐cell receptor (TCR), which consist of two chains, and are surface expressed upon association with invariant CD3 units, which mediate TCR signalling. TCR, especially αβ TCR, exhibit a vast diversity, provided by the somatic rearrangement of TCR gene segments. γδ and αβ T cells exhibit important differences regarding tissue localization and antigen recognition. αβ T cells express as coreceptor CD4 or CD8. CD4+ T cells are MHC class II‐restricted and comprise T helper (T H ) and regulatory (Treg) T cells. CD8αβ+ T cells are MHC class I‐restricted and are predominantly cytotoxic T cells (CTL). γδ T cells constitute as a link between innate and adaptive immunity. They are found frequently in epithelia (e.g. skin and intestine) and recognize nonclassical MHC class Ib molecules and diverse small nonpeptidic compounds in a coreceptor and MHC restriction independent manner.
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