Molecules that can be recognised by the immune system are called [blank_start]antigens[blank_end]. If antigens are distinguished as [blank_start]foreign[blank_end], an [blank_start]immune[blank_end] [blank_start]response[blank_end] is stimulated against them.
Cells of the immune system called [blank_start]B cells[blank_end] produce antibodies against foreign antigens. This targets the [blank_start]antigen[blank_end] for destruction by specialised cells of the immune system. Some B cells will specialise into [blank_start]memory[blank_end] B cells, which remain in the body for long periods of time and are ready to respond [blank_start]quickly[blank_end] if the antigen is seen again.
Vaccines exploit [blank_start]memory[blank_end] immunity, by exposing us to foreign antigens. The antigens alone [blank_start]cannot[blank_end] hurt us, but they do prepare our body for attack by pathogens by stimulating the specialisation of [blank_start]B memory cells[blank_end].
Vaccine have been successfully employed to prevent disease for decades. Poliomyelitis is a disease caused by the polio [blank_start]virus[blank_end]. Vaccines derived from dead an [blank_start]inactivated[blank_end] polioviruses are widespread and have almost eradicated poliomyelitis.