Introduction
Cell Receptors (TCR) Overview
- Function of TCR
- Role in Immunity: T-cell receptors are crucial for cell-mediated immunity, which is the primary function of T-lymphocytes (T-cells).
- Antigen Recognition: TCRs enable T-cells to specifically recognize antigens through a surface receptor that belongs to the immunoglobulin superfamily of proteins.
- Cell Surface Receptor: Unlike antibodies, the TCR is always a membrane protein and never detaches from the cell surface.
- Types of TCR
- TCR1: Found in a small subset of T-cells (<10%) located beneath mucosal epithelia and skin, involved in local immunity. This type is less studied and not the focus here.
- TCR2: The more commonly discussed TCR, present on the majority of T-cells.
- Structure of TCR
- Composition: TCR is a heterodimer consisting of α- and β-chains linked by disulfide bonds.
- Chain Orientation: The N-terminus of each chain is extracellular, while the C-terminus resides within the cytoplasm.
- Glycosylation: Like most membrane proteins, TCR2 is glycosylated to enhance functionality.
- Domain Arrangement: Each chain features two domains:
- Constant Domain: Closer to the cell membrane and remains the same across different TCRs.
- Variable Domain: Located at the N-terminus, varies among T-cells, allowing the recognition of different antigens. This domain contains hypervariable or complementary-determining regions (CDRs), which form the antigen-binding site when paired (V-α and V-β).
- Comparison with Antibodies
- Size: TCRs are considerably smaller than antibodies, analogous to an isolated Fab fragment of an antibody.
- Functionality Difference:
- Antibodies: Can be secreted outside B-cells and act in soluble form. They include an Fc fragment that serves important adaptor functions.
- TCRs: Remain membrane-bound, focusing solely on transmitting a signal inside the cell when an antigen is bound. TCRs lack a large constant part because they do not need to function outside the cell or perform secondary roles like antibodies.