Molecules that can be recognised by the immune system are called . If antigens are distinguished as , an is stimulated against them.

Cells of the immune system called produce antibodies against foreign antigens. This targets the for destruction by specialised cells of the immune system. Some B cells will specialise into B cells, which remain in the body for long periods of time and are ready to respond if the antigen is seen again.

Vaccines exploit immunity, by exposing us to foreign antigens. The antigens alone hurt us, but they do prepare our body for attack by pathogens by stimulating the specialisation of .

Vaccine have been successfully employed to prevent disease for decades. Poliomyelitis is a disease caused by the polio . Vaccines derived from dead an polioviruses are widespread and have almost eradicated poliomyelitis.