T Cells are lymphocytes that form a major component of adaptive cellular immunity. They develop from precursor cells associated with the bone marrow and undergo maturation in the thymus, which gives them their name. Unlike antibodies, T-cell receptors generally recognise antigen-derived peptides only when those peptides are presented by major histocompatibility complex molecules on cell surfaces. T cells therefore specialise in recognising cellularly presented information rather than simply binding freely circulating molecules. Their functions include coordinating immune responses, destroying infected or abnormal cells and regulating the activity of other immune cells.
The T-cell receptor is a membrane-bound antigen-recognition molecule composed most commonly of alpha and beta chains. T-cell receptor genes undergo V(D)J recombination during development, generating extensive receptor diversity. T-cell development in the thymus includes positive selection, which favours cells capable of interacting appropriately with self-MHC molecules, and negative selection, which removes many cells with excessively strong recognition of self antigens. These selection processes contribute to self-tolerance while preserving the ability to recognise foreign or abnormal peptide-MHC complexes. Mature T cells then circulate through blood and lymphoid tissues, where antigen-specific cells can become activated.
CD4 and CD8 identify two major T-cell populations with different but overlapping functions. CD4-positive T cells generally recognise peptides presented by MHC class II molecules and can differentiate into functional subsets that coordinate immune responses. CD8-positive T cells generally recognise peptides presented by MHC class I and can become cytotoxic T lymphocytes capable of killing infected or transformed cells. These categories are not simply equivalent to helper and killer functions in every circumstance, because T-cell biology includes considerable functional diversity. Regulatory T cells, for example, are predominantly CD4-positive and contribute to suppression of excessive immune responses and maintenance of tolerance.
T-cell activation typically requires more than recognition of peptide-MHC alone. A naive T cell generally requires antigen-specific receptor engagement together with appropriate co-stimulatory signals and cytokine cues. Dendritic Cells are particularly important for initiating naive T-cell responses because they can present antigen and provide co-stimulation within lymphoid tissues. Once activated, T cells proliferate through clonal expansion and differentiate into effector and memory populations. The strength, duration and context of signalling influence the resulting phenotype, allowing immune responses to adapt to different types of infection or tissue disturbance.
Helper T-cell differentiation produces populations with specialised cytokine profiles and functional roles. Th1 responses are associated particularly with cellular immunity against intracellular pathogens and activation of macrophage functions. Th2 responses are associated with eosinophils, mast-cell-related responses and defence against many extracellular parasites, while also contributing to allergic disease. Th17 cells produce cytokines involved in epithelial and neutrophil-associated defence, particularly against extracellular bacteria and fungi. Regulatory T cells help limit immune activation and maintain tolerance. These categories are useful models, although actual immune responses can involve plasticity and overlapping characteristics rather than perfectly isolated cell types.
Cytotoxic T lymphocytes kill target cells primarily through mechanisms that induce programmed cell death. They can release perforin and granzymes, which contribute to target-cell apoptosis, and can engage death-receptor pathways under appropriate circumstances. Their recognition is usually specific for peptide-MHC class I complexes, allowing them to identify cells presenting intracellularly derived antigens. This is important in viral infections and tumour surveillance. Cytotoxic T cells differ from Natural Killer Cells in that T cells generally require antigen-specific receptor recognition of a particular peptide-MHC combination, whereas Natural Killer Cells integrate a different set of activating and inhibitory signals and do not require prior antigen-specific receptor rearrangement.
T cells generate immunological memory. Following an immune response, many effector cells disappear during contraction, while a subset survives as memory T cells. Memory populations can respond more rapidly or effectively to subsequent encounters with the relevant antigen. Central memory, effector memory and tissue-resident memory populations differ in location and functional properties. Memory formation contributes to long-term protection after some infections and vaccinations. It is also relevant to immunological pathology because long-lived antigen-specific T cells can contribute to chronic inflammation or autoimmune disease when tolerance is defective.
T-cell dysfunction can contribute to immunodeficiency, autoimmunity, allergy, chronic inflammation and cancer. Excessive or inappropriate T-cell activity can damage host tissues, whereas inadequate T-cell responses can impair control of intracellular infections and malignant cells. Immunological therapies can deliberately modify T-cell activity, including immune checkpoint inhibitors that enhance anti-tumour responses and treatments designed to suppress harmful T-cell activity in autoimmune or transplant-related conditions. Understanding T Cells therefore requires integrating thymic development, antigen presentation, receptor specificity, co-stimulation, differentiation, cytotoxicity, regulation and memory rather than treating them as a single uniform lymphocyte population.
