T-lymphocytes

T-lymphocytes are the antigen-specific effectors of adaptive cellular immunity. They arise from bone marrow-derived precursors that migrate to the thymus, where each cell assembles a unique T-cell receptor by somatic rearrangement of gene segments and is then tested twice: positive selection keeps cells whose receptor binds self major histocompatibility complex (MHC) with modest affinity, and negative selection deletes those binding self peptide too strongly. Survivors leave committed to one MHC class — CD4+ cells read peptide on MHC class II, CD8+ cells on class I — and the cytotoxic T cell and helper lineages diverge accordingly. Recognition alone does not activate a naive cell: it also requires costimulation through CD28 and a cytokine signal, a three-part requirement that prevents responses to antigen encountered without evidence of danger, and that professional antigen-presenting cells, particularly dendritic cell populations, are uniquely equipped to supply.

Activated CD4+ cells differentiate into subsets defined by the cytokines they make and the threats they suit — Th1 against intracellular pathogens, Th2 against helminths, Th17 at barrier surfaces, and the regulatory T cell compartment that restrains all of them. CD8+ cells become killers that engage any cell displaying their peptide on class I. Most contract after the response resolves, leaving memory populations that respond faster and at lower threshold on re-exposure; γδ T cells sit outside this scheme, recognizing non-peptide antigens without MHC restriction. survival through development and in the periphery depends on the balance of Bcl-2 family proteins.

In cancer, T cells are the effectors nearly every immunotherapy is trying to reach. Checkpoint inhibitor therapy works by relieving inhibitory signaling on T cells and can promote the priming and recruitment of new antitumor T-cell responses, and adoptive approaches supply the cells directly — tumor-infiltrating lymphocytes, or receptor-engineered and CAR T cells. Targeting T-cell malignancies with T cells is the hard case, since shared antigens cause the engineered cells to kill each other and to deplete the normal compartment. T cells act throughout in concert with macrophage populations, natural killer cell populations and dendritic cells, and their metabolic state within the tumor microenvironment is now as much a target as their specificity.