Final answer:
The CD4 co-receptor site on MHC Class II molecules interacts with the α1 and β1 domains, crucial for T cell immune responses.
Step-by-step explanation:
The domains of MHC Class II molecules that have the CD4 co-receptor site are α1 and β1 domains. These molecules play a key role in the immune system by presenting antigens to T cells. Both chains of the MHC II molecule span the plasma membrane and fold into separate domains, with one of the chains being the α chain and the other being the β chain. The CD4 co-receptor site specifically interacts with the α1 and β1 domains of the MHC Class II molecule, leading to effective communication between MHC II-presenting cells, like macrophages, dendritic cells, and B cells, and CD4+ T cells, which include helper and regulatory T cells.