T lymphocytes,(also called T cells) form a group of white blood cells that serve the immune defense. T lymphocytes, together with B lymphocytes, represent the acquired (adaptive) immune response. The T in the name stands for the thymus, where the cells mature. They develop from stem cells in the bone marrow and then mature in the thymus. In the thymus, T-cells multiply and differentiate into into different types of t-cells: helper, regulatory, or cytotoxic T-cells or they become memory T-cells.