Final answer:
MHC II molecules are expressed on antigen-presenting cells and play a crucial role in presenting foreign antigens to helper T-cells. The assembly of MHC II involves alpha and beta chains, and the invariant chain prevents peptide binding until the site of extracellular protein breakdown.
Step-by-step explanation:
MHC II molecules are expressed only on the surface of antigen-presenting cells, such as macrophages, dendritic cells, and B cells. They are responsible for presenting processed foreign antigens to helper T-cells for immune response activation. The assembly of MHC II molecules involves the alpha and beta chains, which need to be properly assembled before they can bind to peptides. The invariant chain on MHC II prevents peptides from binding until it reaches the site of extracellular protein breakdown.