The Collaborative International Dictionary of English v.0.48:

T cell \T" cell`\ (t[=e]"s[e^]l`) [From Thymus, the site of
   maturation of T-cells.]
   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.
   [PJC]

WordNet (r) 3.0 (2006):

T cell
    n 1: a small lymphocyte developed in the thymus; it orchestrates
         the immune system's response to infected or malignant cells
         [syn: T cell, T lymphocyte]