Question
T lymphocytes.
Answer
T lymphocytes (T cells) are cells of the adaptive immune system responsible for cell-mediated immunity, deriving from bone-marrow lymphoid progenitors but maturing in the thymus (the “T” originates from this site), where they undergo positive and negative selection to generate a self-tolerant, MHC-restricted repertoire.
Key features:
- Each T cell expresses a unique T-cell receptor (TCR), associated with the CD3 complex, which recognizes antigenic peptide only when presented in the context of a self-MHC molecule (MHC restriction) — unlike B cells, T cells cannot recognize free/soluble antigen directly.
Major subsets:
- CD4+ T-helper cells — recognize antigen on MHC class II, orchestrate the immune response via cytokine secretion, further divided into Th1, Th2, Th17, and T follicular helper subsets.
- CD8+ cytotoxic T lymphocytes — recognize antigen on MHC class I, directly kill infected/abnormal cells via perforin/granzyme-mediated apoptosis, particularly important against intracellular pathogens (viruses) and tumour cells.
- Regulatory T cells (Treg) — CD4+CD25+FoxP3+, suppress excessive immune responses and maintain peripheral self-tolerance.
- Memory T cells — long-lived cells generated after antigen exposure, providing a faster, stronger response on re-exposure.
Clinical significance: T-cell number/function is assessed in various immunodeficiencies (e.g., HIV/AIDS depletes CD4+ T cells, DiGeorge syndrome causes thymic aplasia with absent T-cell development); T cells mediate graft rejection and Type IV (delayed-type) hypersensitivity reactions, and are the target of many immunosuppressive therapies used in transplantation and autoimmune disease.