CD4 primarily binds to a monomorphic region of MHC class II molecules.
Here's a more detailed explanation:
CD4 (Cluster of Differentiation 4) is a glycoprotein found on the surface of immune cells such as helper T cells, regulatory T cells, monocytes, macrophages, and dendritic cells. Its primary function is to assist T cells in interacting with antigen-presenting cells (APCs). This interaction is crucial for initiating an adaptive immune response.
CD4 and MHC Class II Interaction
The core of CD4's function lies in its ability to bind to MHC Class II molecules. MHC Class II molecules are found on the surface of APCs and present processed antigens to T cells. The binding of CD4 to MHC Class II serves several vital purposes:
- Stabilizing the interaction: CD4 acts like molecular glue, strengthening the relatively weak interaction between the T cell receptor (TCR) on the T cell and the MHC Class II molecule on the APC. This increased stability ensures sufficient time for T cell activation.
- Signal Transduction: In addition to stabilization, the binding of CD4 to MHC II initiates signaling pathways inside the T cell, helping to activate it.
- Specificity: This interaction ensures that CD4+ T cells interact with cells presenting antigens via MHC Class II, directing adaptive immune responses towards extracellular pathogens.
Location of Binding
CD4 does not bind to the peptide antigen held within the MHC Class II molecule's groove. Instead, it binds to a specific, unchanging (monomorphic) region of the MHC Class II molecule itself. This region is distinct from the highly variable regions involved in peptide binding, enabling CD4 to interact with all MHC Class II molecules regardless of the presented antigen.
In summary, CD4’s binding to the monomorphic region of MHC Class II molecules is essential for stabilizing T cell-APC interactions and initiating targeted immune responses.
