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Active immunity is defined as immunity to a pathogen that occurs following exposure to all or part of that pathogen. When the body is exposed to a novel disease agent, a cascade of signaling molecules and action from the innate immune system results in the activation of the adaptive immune system. Production of large numbers of T cells and B cells specific to the pathogen is promoted.
Passive immunity, on the other hand, is developed by antibodies that are produced outside and then introduced into the body. This can occur naturally, such as when antibodies are transferred from a mother to her baby through the placenta or breast milk, or artificially, such as when a person is given an injection of antibodies.