An immune cell that learns to recognize specific threats and either kills infected cells directly or coordinates the broader immune response.T lymphocytes mature in the thymus and mediate adaptive cellular immunity. CD4+ helper T cells coordinate immune responses by activating B cells and macrophages; CD8+ cytotoxic T cells directly kill virus-infected or tumor cells. Regulatory T cells (Tregs) suppress excessive immune activation. Several peptide therapeutics (thymosin alpha-1, thymalin) target T cell maturation and function. T cell exhaustion in chronic disease and cancer is a key target of immunotherapy research.