title: T lymphocytes.
abstract: T cells are major players in the adaptive immune response to pathogens. They express clonally distributed, highly polymorphic antigen receptors that enable them to recognize cell-associated antigen. Upon antigen recognition, T cells undergo clonal amplification and progressively acquire effector functions, ranging from the production of paracrine soluble factors that provide "help" to other immune cells to the ability to kill pathogen-infected cells with surgical precision. A pool of antigen-reactive T cells reverts to a state of quiescence and maintains a long-lasting memory of antigen encounter. T cells develop in the thymus through a rigorous selection process that recapitulates Darwinian phylogenesis: only the "fittest" survive, i.e. those that can efficiently recognize infectious non-self-antigens but ignore, or are silenced, by non-infectious self-antigens. Due to their ability to discriminate between self and non-self, T cells are the major effectors of allograft rejection. T cells are involved in the pathogenesis of several human disorders, resulting from their defective or dysregulated function. The former leads to a severe state of immunodeficiency, the latter to organ-specific or systemic autoimmunity.
authors.affiliation: Unit of Leukocyte Biology, Vita-Salute San Raffaele University School of Medicine, DIBIT-Scientific Institute San Raffaele, Via Olgettina, 58, 20132 Milano, Italy. fabbri.monica@hsr.it
authors.forename: Monica
authors.initials: M
authors.lastname: Fabbri
authors.forename: Chanel
authors.initials: C
authors.lastname: Smart
authors.forename: Ruggero
authors.initials: R
authors.lastname: Pardi
mesh_terms: D000163:Acquired Immunodeficiency Syndrome / Q000276:immunology; D001327:Autoimmune Diseases / Q000276:immunology; D006801:Humans; D007156:Immunologic Memory; D016511:Severe Combined Immunodeficiency / Q000276:immunology; D013601:T-Lymphocytes / Q000276:immunology* / Q000648:ultrastructure