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Passive immunity is a form of immunity which occurs when antibodies are transferred from one person to another individual, or when antibodies of animal origin are introduced to a human. This type of immunity is short acting, and is typically seen in cases where a patient needs immediate protection from something and he or she cannot form antibodies quickly enough independently.
In natural passive immunity, antibodies are passed from a mother to a child. Antibodies can be transferred through the placenta, or transmitted through the colostrum, a liquid which is produced in the breasts for a baby's first meal. The antibodies transmitted through the colostrum and placenta generally only last for several weeks, which is long enough to allow the baby to start to build up its own immune system and to make its own antibodies.
Artificial passive immunity involves the introduction of antibodies through means such as injection. For example, in the treatment of some diseases, patients may be given a serum derived from patients who have recovered to help them fight the disease. This practice is sometimes seen when people are dealing with an outbreak of a new or extremely virulent disease for which no known treatment is available.
Prophylactic treatments of antibodies are sometimes given when people are exposed to diseases like rabies, botulism, tetanus, and diphtheria. By giving a patient passive immunity, a doctor can help the patient recover from a disease which the patient's body is unable to fight. This type of immunity is not the same thing as vaccination, a process in which small amounts of antigens are introduced to the body to encourage it to form its own antibodies so that it can fight those antigens in the future.
When the body makes its own antigens, as seen when someone is vaccinated, this is known as active immunity. Active immunity lasts much longer than passive immunity, sometimes persisting over a lifetime to keep someone from getting sick, in marked contrast with the temporary state of passive immunity. Inducing active immunity is often preferred, when possible, because it will support the patient's health in the long term. However, artificially creating active immunity is very complicated, as it involved the controlled introduction of antigens in the body, and this can be dangerous for some patients, as seen when so-called "live" vaccines cause outbreaks.