T cell receptor

T cell receptor (TCR) is a molecule found on the surface of T lymphocytes (or T cells), which are a type of white blood cells. These receptors are critical for the immune system's ability to recognize and respond to antigens, which are specific parts of pathogens like viruses and bacteria. The TCR is responsible for recognizing fragments of antigens as peptides bound to major histocompatibility complex (MHC) molecules on the surface of antigen-presenting cells. This recognition is a key part of the adaptive immune response, allowing T cells to target and respond to specific pathogens.

Structure

The T cell receptor is a heterodimer, consisting of two different protein chains, which are typically either α and β chains or, less commonly, γ and δ chains. These chains are connected by a disulfide bond and each chain has a variable (V) and a constant (C) region. The variable region is responsible for recognizing antigens. The diversity of the TCR is generated through a process called V(D)J recombination, which allows a vast array of antigen specificities.

Function

Upon recognition of a peptide-MHC complex, the TCR transmits a signal into the T cell, leading to T cell activation. This activation process often requires additional signals from co-stimulatory molecules and cytokines. Once activated, T cells can differentiate into various subtypes, such as helper T cells, cytotoxic T cells, and regulatory T cells, each playing a unique role in the immune response. Helper T cells assist other white blood cells in immunologic processes, including the activation of cytotoxic T cells and macrophages. Cytotoxic T cells are involved in directly killing infected cells, while regulatory T cells play a role in suppressing immune responses.

Types of T Cell Receptors

There are two main types of TCRs, based on the chains they are composed of:

• αβ T cell receptors: These are the most common type and are found on the majority of peripheral T cells. They typically recognize peptides presented by MHC class I and MHC class II molecules.
• γδ T cell receptors: These are less common and are found on a small subset of T cells. γδ T cells can recognize a wide variety of antigens without the need for peptide presentation by MHC molecules, including some lipids and small-molecule metabolites.

Clinical Significance

The specificity of the TCR for its antigen is the basis for the immune system's ability to detect and respond to a wide array of pathogens. However, this specificity can also lead to autoimmune diseases if T cells mistakenly recognize self-antigens as foreign. Additionally, the ability to engineer T cells with specific TCRs or chimeric antigen receptors (CARs) is a basis for novel cancer therapies, such as CAR T-cell therapy, which involves modifying a patient's T cells to specifically target tumor cells.

See Also

• Major histocompatibility complex
• Immunology
• Adaptive immune system
• Antigen-presenting cell
• V(D)J recombination

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