T cell
T cell, type of white blood cell (leukocyte) that is an essential part of the immune system. T cells are one of two primary types of lymphocytes—B cells being the second type—that determine the specificity of immune response to antigens (foreign substances) in the body.
- Helper T cell (Th, or CD4+ cell): Produces factors that activate nearly all other immune system cells, including B cells and cytotoxic T cells.
- Regulatory T cell (Treg): Specialized subset of CD4+ T cells that serves a crucial role in suppressing inflammatory immune responses and maintaining immune tolerance, thereby preventing autoimmunity.
- Cytotoxic T cell (killer T cell, or CD8+ T cell): Destroys cells with foreign or abnormal antigens on their surfaces, including tumor cells and cells carrying infectious agents.
- Memory T cell: Responds to specific antigens that the immune system has previously encountered, providing long-lasting immune defense against reinfection.
T cells originate in the bone marrow and mature in the thymus. In the thymus, T cells multiply and differentiate into helper, regulatory, or cytotoxic T cells or become memory T cells. They are then sent to peripheral tissues or circulate in the blood or lymphatic system. Once stimulated by the appropriate antigen, helper T cells secrete chemical messengers called cytokines, which stimulate the differentiation of B cells into plasma cells (antibody-producing cells). Regulatory T cells, which are a specialized subset of CD4+ T cells, act to control immune reactions, hence their name. They have a crucial role in suppressing inflammatory immune responses and maintaining immune tolerance, thereby preventing autoimmunity (immune reaction against the body’s own components). Cytotoxic T cells, which are activated by various cytokines, bind to and kill infected cells and cancer cells.
Subsets of T cells also exist and in some instances serve as a crucial link between the innate and adaptive immune systems by rapidly responding to infections and influencing subsequent adaptive immune responses. Natural killer T cells (NKT cells), for example, rapidly recognize and destroy infectious agents, while memory T cells mount a slower initial reaction, enabling a faster and stronger attack upon re-exposure. Gamma delta T cells likewise respond quickly to foreign antigens, while also promoting specific long-lasting immune responses through the release of cytokines that activate B cells, cytotoxic T cells, and other adaptive immune cells.
Each T and B cell expresses a unique antigen receptor, formed through a process known as V(D)J recombination, which involves the random recombination of variable (V), diversity (D), and joining (J) gene segments. Because the body contains millions of T and B cells, it can respond to virtually any antigen.