T cell

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of the lymphocytes that mature in the thymus and have the ability to recognize specific peptide antigens through the receptors on their cell surface.

from the GNU version of the Collaborative International Dictionary of English.

• A type of white blood cell that circulates in the blood and lymph, and provides cell-mediated immunity for the organism, protecting against infecting cells or the body's own malignant cells; also called T lymphocyte. There are several types of T cells. They develop, as do B cells, from progenitor cells in the bone marrow, but are distinguished from B-cells (B-lymphocytes) by their site of differentiation; T-cells mature in the thymus and B-cells in the bone marrow (in birds in the Bursa of Fabricius). They also have different antigen receptors from those of B-cells. T-cells differentiate into cells that can directly kill infecting cells (cell-mediated immunity, cytotoxity) or activate other cells of the immune system (helper T cells), whereas B-cells differentiate on activation into antibody-secreting plasma cells. Helper T cells interact with B-cells by secreting lymphokines that stimulate the B cell which have detected a foreign antigen to enter the cell cycle and develop, by repeated mitosis, into a clone of cells with identical receptors, and then to secrete antibodies to that specific antigen.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun immunology A lymphocyte, from the thymus, that can recognise specific antigens and can activate or deactivate other immune cells.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a small lymphocyte developed in the thymus; it orchestrates the immune system's response to infected or malignant cells