When people or animals are exposed to a disease-causing organism by natural means or vaccination, the organism or a part of it interacts with the cells of the animal's immune system. And the body activates cells which can kill the disease-causing organism more directly. Examples of passive immunity include the antibodies received by a fetus through the placenta, antibodies the newborn receives from its mother through colostrum, antivenins to treat snakebite, and bone marrow transplants which help replace the lymphcytes. As the antibodies the animal received break down through natural aging, or are used up destroying disease-causing organisms, the animal's body cannot replace them. However, in the case of active immunity, more antibodies are produced whenever the immune system comes in contact with the same organism again.
Active immunity is self-perpetuating.
Passive immunity is not. Two types of passive immunity protect young kittens and puppies. It must be noted that the kitten or puppy will only receive antibodies against diseases for which the mother had been recently vaccinated against or exposed to. As an example, a bitch that had NOT been vaccinated against or exposed to parvovirus , would not have any antibodies against parvovirus to pass along to her puppies. The puppies then would be susceptible to developing a parvovirus infection.