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T cells are developed from lymphoid stem cells in the bone marrow and mature in the thymus. Cell-mediated immunity is primarily carried out by T cells. Subsequent differentiation of T cells produces cytotoxic (killer) T cells, delayed-hypersensitivity T cells, helper T cells, and regulatory T cells. T cells act in situations where antigens are embedded in cell membranes or are inside host cells and thus are inaccessible to antibodies.