Whenever T cells and B cells are activated, some become "memory" cells. The next time that an individual encounters that same antigen, the immune system is primed to destroy it quickly. This is active immunity because the body's immune system prepares itself for future challenges. Long-term active immunity can be naturally acquired by infection or artificially acquired by vaccines made from infectious agents that have been inactivated or, more commonly, from minute portions of the microbe.

Short-term passive immunity can be transferred artificially from one individual to another via antibody-rich serum; similarly, a mother enables an infant to naturally acquire protection while growing within her by donating her antibodies and certain immune cells. This is passive immunity because the infant who is protected does not produce antibodies, but borrows them.

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