Question

Which molecule stabilizes the interaction between T cell receptor and MHC class II?

• A. CD4
• B. CD8
• C. CD28
• D. CD40

Hint:

Use the "Rule of 8" to remember TCR-MHC associations: CD\textbf{4} $\times$ MHC \textbf{II} = 8, and CD\textbf{8} $\times$ MHC \textbf{I} = 8.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
T cells recognize antigens presented by Major Histocompatibility Complex (MHC) molecules using their T cell receptor (TCR).
To ensure specific and highly stable binding, T cells utilize distinct co-receptors that bind to invariant regions of the MHC molecules.

Step 2: Detailed Explanation:
Let's examine the role of each given molecule.
- CD4: This is a co-receptor expressed primarily on helper T cells.
It specifically binds to the invariant $\beta_2$ domain of MHC class II molecules.
This interaction physically stabilizes the TCR-MHC II complex and enhances the intracellular signaling cascade.
- CD8: This is a co-receptor expressed on cytotoxic T cells that binds to MHC class I molecules.
- CD28: This is a costimulatory receptor on T cells that binds to B7 molecules on antigen-presenting cells to provide Signal 2.
- CD40: This molecule is found on antigen-presenting cells and interacts with CD40L on T cells to facilitate mutual activation.

Step 3: Final Answer:
Because CD4 specifically binds and stabilizes MHC class II interactions, it is the correct answer.
Therefore, the correct option is (A).