T cells, also called T lymphocytes, are important components of the adaptive immune system. Production starts from the hematopoietic stem cells in the bone marrow, from which T-cell progenitor cells arise. These cells migrate to the thymus for further maturation. A functional mature T cell develops from a step-by-step process creating a T-cell receptor (TCR) complex, selecting T cells with appropriate affinity to self-antigens associated with major histocompatibility molecules (positive selection), and expressing either CD4 or CD8. In this series, cells predisposed to autoimmunity undergo apoptosis (negative selection). When released from the thymus, the naive mature T cells travel to the secondary lymphoid organs for activation. Two signals, an antigen-specific binding of TCR and costimulation, are required to be activated (ready to mount an immune response). In the case of CD8+ T cells, additional cytokine stimulation is necessary. Depending on the cytokines they are exposed to during antigen stimulation, the undifferentiated mature T cell (Th0) develops into cells with different functions: CD4+ become T helper (Th) cells and CD8+ become cytotoxic, or cytolytic, cells. Th cells have other subtypes; the most characterized are Th1, Th2, Th17, follicular Th cells, and regulatory T cells. Other types include natural killer T cells and memory T cells. These mature differentiated T cells ensure effective surveillance and immediate response to pathogens, tumor cells, and foreign tissues and provide immunologic memory.