Final answer:
MHC II molecules present exogenous antigens to helper T cells, initiating an immune response.
Step-by-step explanation:
MHC II molecules present exogenous antigens. Exogenous antigens are antigens that are derived from sources outside the body, such as bacteria, viruses, and other foreign substances. MHC II molecules are only found on the surface of antigen-presenting cells (macrophages, dendritic cells, and B cells) and are responsible for presenting these exogenous antigens to helper T cells, initiating an immune response.