T lymphocyte

T lymphocyte (also known as a T cell) is a type of white blood cell that is a key component of the immune system. T cells are named after the thymus, an organ situated beneath the breastbone, where they mature.

Function

T cells are responsible for a variety of immune responses in the body. They play a crucial role in cell-mediated immunity, a type of immune response that involves the activation of phagocytes, antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to an antigen.

Types of T cells

There are several types of T cells, each with a different function:

• Helper T cells (Th cells) - These cells assist other white blood cells in immunologic processes, including maturation of B cells into plasma cells and memory B cells, and activation of cytotoxic T cells and macrophages.

• Cytotoxic T cells (Tc cells) - These cells are capable of killing cancer cells, cells that are infected (particularly with viruses), or cells that are damaged in other ways.

• Memory T cells - These are a subset of antigen-specific T cells that persist long-term after an infection has resolved. They quickly expand to large numbers of effector T cells upon re-exposure to their cognate antigen, thus providing the immune system with "memory" against past infections.

• Regulatory T cells (Treg cells) - These cells are crucial for the maintenance of immunological tolerance. Their major role is to shut down T cell-mediated immunity toward the end of an immune reaction and to suppress auto-reactive T cells that escaped the process of negative selection in the thymus.

T cell activation

T cells are activated when they are presented with peptide antigens by MHC class II molecules, which are expressed on the surface of antigen-presenting cells (APCs). Once activated, they divide rapidly and secrete small proteins called cytokines that regulate or assist in the active immune response.

Clinical significance

T cells play a crucial role in the body's defense against disease. They are the target of several pathogens, notably HIV, which can lead to the progression of AIDS if the T cells are destroyed. In addition, T cell dysfunction can lead to the development of autoimmune diseases.