T-Cell Receptor Complexes

T-cell receptors (TCRs) are membrane-bound receptor proteins found on the surface of T lymphocytes. They are responsible for recognizing antigenic peptides presented by major histocompatibility complex (MHC) molecules, initiating the T cell immune response. TCRs have a heterodimeric structure consisting of two polypeptide chains: α and β chains (in αβ T cells) or γ and δ chains (in γδ T cells).

Each TCR chain has several domains:

1. Variable (V) domain: This is the most diverse region and is responsible for antigen recognition. It contains hypervariable regions called complementarity-determining regions (CDRs) that directly interact with the antigen peptide.
2. Constant (C) domain: This domain is less variable and provides structural support.
3. Hinge region: A flexible region connecting the C domain to the transmembrane domain.
4. Transmembrane domain: A hydrophobic region that anchors the TCR in the T cell membrane.
5. Cytoplasmic tail: A short intracellular region that associates with signal transduction molecules like CD3.

The α and β (or γ and δ) chains are assembled together via a disulfide bond to form the complete TCR heterodimer. Each chain contains one V and one C domain.

TCR CD3 complex:

Additionally, the TCR associates with the CD3 complex, which consists of four invariant polypeptide chains (γ, δ, ε, and ζ). The CD3 chains also have extracellular immunoglobulin-like domains, a transmembrane region, and cytoplasmic tails involved in initiating intracellular signaling cascades upon TCR engagement.

The majority of T cells express αβ TCRs, while a minority express γδ TCRs. The γδ TCRs have a similar overall structure but recognize different antigens in an MHC-independent manner.

In summary, TCRs exhibit a multidomain, heterodimeric structure with variable antigen-binding regions and associated CD3 signaling subunits, enabling highly diverse yet specific antigen recognition critical for T cell functions.

Next Topic: B-Cell Receptor Complexes

Source: Claude 3 Sonnet response prompted and edited by Joel Graff.

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