Immunity is a state in which the body is protected from infectious disease. It is conferred by the immune system, a complex network of cells, tissues and chemicals that fight infection and kill organisms when they invade the body. There are three categories of immune protection, all of which help protect the body from infectious diseases. It can be innate or acquired, active or passive, and natural or artificial. These categories can mix and match to produce, for example, natural passive or artificial passive immune protection.
The category of innate or acquired protection refers to the type of immune response that is mounted by the immune system. An innate immune response is not specific to the pathogen to which the system is responding, and it happens almost immediately when an infectious organism invades the body. In contrast, an acquired immune response is specific to the pathogen and can take several days to build up. The acquired immune response also involves the development of immunological memory, a state in which the immune system can quickly mount a response to an infectious organism that it has previously encountered.
Active or passive immune protection is determined by the way in which the protection is conferred. Protection that is active is conferred by contact with an infectious organism or a vaccine. This provokes an active immune response in the person who comes into contact with the organism. Passive immunity refers to the fact that an individual is protected, even though his or her immune system has not itself mounted a response. For example, the trans-placental transfer of antibodies from mother to child is a type of passive immune protection. Another example is the transfer of antibodies from mother to child in breast milk.
The third category, natural or artificial immunity, refers to whether the protection has developed with or without intervention. For example, trans-placental antibody transfer is a natural process, because it has occurred solely though an interaction between mother and fetus. If, after the baby was born, an antibody injection was administered, this would be an example of artificial protection, because the antibodies have been removed from one individual, purified, then injected into another. Vaccination is another example of artificial immune protection and is also an example of active acquired protection.
Vaccination and passive immune treatments are not the only ways to confer immune protection. It can be improved in many other ways, as has been demonstrated over the course of history. For example, improvements in sanitation, diet and pest control have contributed to the reduced severity of diseases and the increased life expectancy that people in developed countries enjoy now, compared to that of several hundred years ago.