Final answer:
CD4 binds to the antigen-binding cleft of MHC class II, comprised of α1 and β₁ domains, which is essential for the interaction between CD4+ T cells and antigen-presenting cells.
Step-by-step explanation:
The CD4 molecule binds to the major histocompatibility complex (MHC) class II on antigen-presenting cells. Specifically, CD4 interacts with the antigen-binding cleft formed by the α1 and β₁ domains of the MHC class II molecule. This interaction takes place when a CD4+ T cell recognizes an extracellular pathogen that has been processed and presented by an antigen-presenting cell (APC) such as macrophages, dendritic cells, and B cells. This binding is crucial for the immune response as it helps keep the T cell in close contact with the APC, facilitating the T cell receptor (TCR) binding to antigens and subsequent activation of the T cell.