Which receptor on the surface of the helper T cell interacts with MHC class II molecules?

The correct choice is CD4 because it specifically interacts with MHC class II molecules presented by antigen-presenting cells (APCs). MHC class II molecules are primarily expressed on professional APCs such as dendritic cells, macrophages, and B cells, and they play a crucial role in presenting antigens to helper T cells.

When the CD4 receptor on the surface of a helper T cell binds to the MHC class II molecule, this interaction is essential for both the activation of the helper T cell and the subsequent immune response. The binding of CD4 to MHC class II also helps stabilize the interaction between the T-cell receptor and the peptide-MHC complex, leading to a more robust activation signal.

CD8, on the other hand, is a receptor found on cytotoxic T cells and interacts with MHC class I molecules, which are present on all nucleated cells. The T-cell receptor, while critical for recognizing antigens presented by both MHC class I and II, does not specifically interact with MHC class II itself in the way that CD4 does. CD28 is a co-stimulatory molecule that provides an additional signal required for T cell activation but does not have a direct functional interaction with MHC class II. Thus