Natural active: Occurs during infection. It is active because lymphocytes are activated by antigens on pathogen's surface.

Artificial active: Injecting or taking antigens by mouth. Takes time for T and B cells to be activated but gives long lasting immunity.

Natural passive: Mother to child through placenta or milk.

Artificial passive: Used during potentially fatal diseases. Provides an instant response but only temporary as antibodies are not the body's own so memory cells are not created. E.g. tetanus - injection of antitoxins given.

Memory cells are only produced in active immunity.
Protection for active immunity is permanent whereas in passive immunity it is only temporary.
Antigens are only encountered in active immunity.
Active immunity takes several weeks to become active but passive is immediate.