T-cell

a type of white blood cell (LYMPHOCYTE) that forms in the bone marrow and moves to the thymus gland, situated in the thorax. There are a whole variety of T-cells involved in recognizing particular foreign bodies (ANTIGENS), and they are important in fighting viral infections and destroying bacteria that have penetrated the cells of the body.

Taken from Dictionary of Science

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T cells or T lymphocytes belong to a group of white blood cells known as lymphocytes, and play a central role in cell-mediated immunity. They can be distinguished from other lymphocyte types, such as B cells and natural killer cells by the presence of a special receptor on their cell surface called T cell receptors (TCR). The abbreviation T, in T cell, stands for thymus, since this is the principal organ responsible for the T cell's maturation. Several different subsets of T cells have been discovered, each with a distinct function.

Taken from Wikipedia