A T cell, or T lymphocyte, is a type of white blood cell belonging to the immune system. T cells are central components of adaptive immunity, providing antigen-specific defense through direct cellular activity and coordination of other immune cells. Their name refers to the thymus, where their precursors undergo development and selection. Unlike B cells, which can differentiate into antibody-secreting cells, T cells act principally through cell-to-cell interactions and signaling molecules. Their functions include killing infected cells, supporting antibody responses, and maintaining tolerance to the body’s own tissues. (ncbi.nlm.nih.gov)
Development and selection
T-cell precursors originate from blood-forming progenitors in the bone marrow and migrate to the thymus. There, developmental signals direct them toward the T-cell lineage. Rearrangement of antigen-receptor gene segments creates a diverse repertoire of receptors, enabling different T cells to recognize different molecular targets. Developing conventional T cells pass through stages defined partly by expression of the surface molecules CD4 and CD8, including a stage in which both are expressed. (ncbi.nlm.nih.gov)
Thymic selection shapes this repertoire. Positive selection preserves cells whose receptors can interact appropriately with the body’s own major histocompatibility complex (MHC) molecules. Negative selection removes many cells that recognize self-antigens too strongly, commonly through apoptosis. These processes establish MHC restriction and contribute to central immune tolerance. Selection is not absolute: additional safeguards outside the thymus control self-reactive cells that escape elimination. Mature cells leave the thymus and populate peripheral lymphoid organs. (ncbi.nlm.nih.gov)
Antigen recognition
The defining antigen-recognition structure is the T-cell receptor (TCR). Most T cells express a receptor composed of alpha and beta chains; a smaller population expresses gamma and delta chains. The TCR operates with the CD3 signaling complex, which transmits receptor-associated signals into the cell. Receptor recognition and intracellular signaling are therefore distinct but coupled functions. (ncbi.nlm.nih.gov)
Conventional alpha-beta T cells recognize fragments of antigens displayed by MHC molecules, rather than freely circulating intact antigens. CD8 generally assists recognition of peptide–MHC class I complexes, whereas CD4 assists recognition of peptide–MHC class II complexes. Class I presentation enables surveillance of intracellular proteins; class II presentation supports interactions with specialized immune cells that process material taken up from their surroundings. The receptor recognizes the combined peptide–MHC surface, not simply the peptide in isolation. (ncbi.nlm.nih.gov)
Activation and clonal expansion
A mature T cell that has not yet encountered its activating antigen is described as naive. Initial activation typically occurs in peripheral lymphoid organs after contact with an antigen-presenting cell, particularly a dendritic cell. TCR recognition supplies an antigen-specific signal, but effective activation of naive cells usually also requires costimulation, notably interaction between CD28 and B7 molecules. Recognition without adequate supporting signals can cause inactivation or deletion rather than a productive response. (ncbi.nlm.nih.gov)
The local cytokine environment influences subsequent differentiation. Activated cells produce growth-promoting signals, including interleukin-2, and increase expression of cytokine receptors. They then undergo clonal expansion, generating numerous descendants with the original receptor specificity. These descendants differentiate into effector cells capable of acting against the antigen-bearing target or helping other immune cells. Expansion is followed by contraction, during which many effector cells die as the response subsides. (ncbi.nlm.nih.gov)
Major functional populations
CD4 helper T cells coordinate immune responses through secreted cytokines and contact-dependent signals. Different helper populations support different activities, including activation of macrophages and assistance to B cells. T-cell help promotes B-cell proliferation and differentiation and shapes production of antibodies. Helper cells therefore connect cellular defense with antibody-mediated immunity rather than representing an entirely separate system. (ncbi.nlm.nih.gov)
CD8 cytotoxic T cells recognize appropriate antigen-bearing targets and can induce their death. Their killing machinery includes granules containing perforin and granzymes, which promote apoptosis in the target cell. Another mechanism uses death-receptor interactions, including Fas and its ligand. Release of cytotoxic granules is directed toward the contact site, concentrating the response on the recognized cell. (ncbi.nlm.nih.gov)
Regulatory T cells suppress immune responses and help prevent inappropriate reactions against self. A major regulatory population depends on the transcriptional regulator FOXP3 for its development and function. Regulatory activity complements deletion and functional inactivation in maintaining peripheral immune tolerance. T-cell populations are thus distinguished not only by what they recognize, but also by the biological consequences of their response. (ncbi.nlm.nih.gov)
Memory and medical significance
Some antigen-experienced T cells persist as memory cells. They contribute to immunological memory by enabling accelerated responses when the relevant antigen is encountered again. Memory formation and maintenance are important subjects in research on infection and vaccines, alongside the corresponding roles of B-cell memory and antibodies. (ncbi.nlm.nih.gov)
Loss or dysfunction of T cells can impair immune defense. HIV, for example, infects and destroys CD4 T lymphocytes, damaging immune function. Conversely, inadequate control of self-reactive T cells can contribute to autoimmunity. These consequences reflect the need to balance antigen responsiveness with mechanisms that prevent injury to healthy tissue. (hivinfo.nih.gov)
T cells are also used in cancer immunotherapy. Tumor-infiltrating lymphocyte therapy expands selected tumor-reactive cells outside the body before reinfusion. CAR T-cell therapy genetically modifies T cells to express chimeric antigen receptors that recognize selected surface targets. These engineered receptors redirect cellular activity toward antigen-bearing cells; treatment can also cause serious adverse effects, including cytokine release syndrome and damage to normal cells sharing the target antigen. (cancer.gov)