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Biology LibreTexts

13. 53: Immunity

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  • Is giving shots to young children a good thing?

    Many, if not most, children hated going to the doctor, as it often meant getting a shot. Why? The shot actually contained a weakened or dead pathogen. And putting some of that dead pathogen into you was a good thing.


    Memory B and T cells help protect the body from re-infection by pathogens that infected the body in the past. Being able to resist a pathogen in this way is called immunity. Immunity can be active or passive.

    Active Immunity

    Active immunity results when an immune response to a pathogen produces memory cells. As long as the memory cells survive, the pathogen will be unable to cause a serious infection in the body. Some memory cells last for a lifetime and confer permanent immunity.

    Active immunity can also result from immunization. Immunization is the deliberate exposure of a person to a pathogen in order to provoke an immune response and the formation of memory cells specific to that pathogen. The pathogen is often injected. However, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is typically used. This causes an immune response without making the immunized person sick. This is how you most likely became immune to measles, mumps, and chicken pox.

    Passive Immunity

    Passive immunity results when antibodies are transferred to a person who has never been exposed to the pathogen. Passive immunity lasts only as long as the antibodies survive in body fluids. This is usually between a few days and a few months. Passive immunity may be acquired by a fetus through its mother’s blood. It may also be acquired by an infant though the mother’s breast milk. Older children and adults can acquire passive immunity through the injection of antibodies.


    • Immunity is the ability to resist infection by a pathogen.
    • Active immunity results from an immune response to a pathogen and the formation of memory cells.
    • Passive immunity results from the transfer of antibodies to a person who has not been exposed to the pathogen.


    1. What is immunity? What role do memory cells play in immunity?
    2. How is active immunity different from passive immunity? Why does active immunity last longer?
    3. Explain how immunization prevents a disease such as measles, which is caused by a virus.