T Cell (T Lymphocyte)
A type of white blood cell central to adaptive immunity. T cells mature in the thymus (hence T cell) and include helper T cells, cytotoxic T cells, and regulatory T cells. Cyclosporine suppresses T cell activation by inhibiting calcineurin. Thymosin alpha-1 promotes T cell maturation.
Technical Context
T cells mature in the thymus (positive selection — selecting T cells that can recognise self-MHC; negative selection — eliminating T cells that react too strongly to self-antigens — establishing central tolerance). T cell activation requires: signal 1 (TCR recognition of peptide-MHC complex on antigen-presenting cell), signal 2 (co-stimulation via CD28-B7 interaction), and signal 3 (cytokine environment directing differentiation). Cyclosporine acts at signal 3: by blocking calcineurin-NFAT pathway → preventing IL-2 transcription → inhibiting T cell proliferation and effector function. This mechanism targets the adaptive immune response while largely sparing innate immunity. Thymosin alpha-1 promotes T cell maturation and function by: stimulating TLR2 and TLR9 on dendritic cells → enhanced antigen presentation → improved T cell priming; and directly promoting T cell differentiation markers (CD4, CD8, CD25 expression). The opposing effects of cyclosporine (suppressing T cells) and thymosin alpha-1 (enhancing T cells) illustrate the different therapeutic needs of immunosuppression vs immunoenhancement.