Tcell

T cells are a type of lymphocyte that mediates cell‑mediated immunity and coordinates adaptive responses. They originate in the bone marrow and mature in the thymus. Mature T cells express the T-cell receptor (TCR) with the CD3 complex and either the CD4 or CD8 co-receptor.

During development, TCRs are generated by V(D)J recombination to create a diverse repertoire. Positive selection in the thymus preserves cells that recognize self‑MHC with low affinity, while negative selection removes self‑reactive clones, helping establish self-tolerance.

Naive T cells are activated when antigen-presenting cells display peptide‑MHC complexes and provide co-stimulatory signals, such as CD28 binding B7. Activation leads to clonal expansion and differentiation into effector and memory cells.

Helper T cells (CD4+) coordinate immune responses by cytokine secretion and assistance to B cells and other immune cells. Cytotoxic T cells (CD8+) kill infected or malignant cells via perforin and granzymes. Regulatory T cells (often CD4+ CD25+ FoxP3+) suppress responses to maintain tolerance and prevent autoimmunity.

Subsets of CD4+ T cells include Th1, Th2, Th17, and T follicular helper cells, defined by cytokine profiles and functions. Memory T cells provide lasting immunity upon re-exposure.

Clinical relevance: T cells are central to infections, cancer, and autoimmunity. HIV targets CD4+ T cells; therapies include CAR‑T cell therapy and immune checkpoint inhibitors that reinvigorate T‑cell responses.