Just as medieval fortresses, such as the Dover Castle, used walls and fortifications to defend their territory, the immune system constantly battles against foreign invaders such as viruses, bacteria, and parasites to defend the organism. Antibody molecules provide a key element in the immune system’s defensive arsenal. For example, specific antibodies can bind to molecules on the surfaces of viruses and prevent the viruses from infecting cells. Above right, an antibody binds to one subunit on hemagglutinin from the surface of influenza virus.