Tcells

T cells, or thymus-derived lymphocytes, are a subset of white blood cells that orchestrate the adaptive immune response. They develop in the thymus from precursor stem cells, undergoing T cell receptor gene rearrangement to form diverse receptors. During maturation, thymocytes undergo positive selection for recognition of self-MHC and negative selection to eliminate self-reactive cells, resulting in a repertoire capable of recognizing peptide antigens presented by major histocompatibility complex (MHC) molecules.

Two major functional subsets are CD4+ helper T cells and CD8+ cytotoxic T cells. CD4+ T cells coordinate immune responses by secreting cytokines and supporting B cell antibody production, macrophage activation, and cytotoxic T cell priming. CD8+ T cells recognize peptides presented by MHC class I molecules and can kill infected or malignant cells through perforin/granzyme pathways or Fas-FasL interactions. Regulatory T cells (often CD4+CD25+FOXP3+) help maintain tolerance and limit immune responses. Memory T cells persist after infection to respond rapidly on re-exposure. Activation typically requires antigen presentation by professional APCs and a second costimulatory signal (for example, CD28–B7).

In health, T cells contribute to defense against pathogens and tumors; dysregulation can lead to autoimmune disease or immune deficiency. Therapeutically, T cells are central to cancer treatments such as chimeric antigen receptor (CAR) T cell therapy and are analyzed by flow cytometry using surface markers. T cell dysfunction, including exhaustion in chronic infection or cancer, remains an area of active research.