Humans develop their adaptive immunity by several general routes. These categories are based on the source of the immunity and whether the protection is produced by one’s own immune system or whether it is received from another person. This classification system is a useful medical guide that can rapidly identify and explain the sources of immunity.
Defining Categories by Mode of Acquisition
Active immunity occurs when an individual receives an immune stimulus (microbe) that activates specific lymphocytes, causing an immune response such as production of antibodies. Active immunity is marked by several characteristics: (1) it is an essential attribute of an immunocompetent individual; (2) it creates a memory that renders the person ready for quick action upon reexposure to that same microbe; (3) it requires several days to develop; and (4) it can last for a relatively long time, sometimes for life. Active immunity can be stimulated by natural or artificial means.
Passive immunity occurs when an individual receives immune substances (primarily antibodies) that were produced actively by the immune system of another human or animal donor. Recipients are protected for a time even though they have not had actual exposure to the microbe. It is characterized by (1) lack of memory for the original antigen, (2) no production of new antibodies against that disease, (3) immediate onset of protection, and (4) short-term effectiveness because antibodies have a limited period of function. Ultimately, the recipient’s body disposes of them. Passive immunity can also be natural or artificial in origin.
Natural immunity encompasses immunity that is acquired during any normal biological experiences of an individual but not through medical intervention.
Artificial immunity is protection from infection obtained through medical procedures. This type of immunity is induced by immunization with vaccines and immune serum.
Putting together all combinations of these categories, we arrive at four possible examples of origins of immunity.
Natural Active Immunity: Getting an Infection
After recovering from infectious disease, a person may be actively resistant to reinfection for a period that varies according to the disease. In the case of childhood viral infections such as measles, mumps, and rubella, this natural active stimulus provides nearly lifelong immunity. Other diseases result in a less extended immunity of a few months to years (such as pneumococcal pneumonia and shigellosis), and reinfection is possible. Even a subclinical infection can stimulate natural active immunity. This probably accounts for the fact that some people are immune to an infectious agent without ever having been noticeably infected with or vaccinated for it.
Natural Passive Immunity: Mother to Child
Natural, passively acquired immunity occurs as a result of the prenatal and postnatal, mother–child relationship. During fetal life, IgG antibodies circulating in the maternal bloodstream are small enough to pass or be actively transported across the placenta. Anti bodies against tetanus, diphtheria, pertussis, and several viruses regularly cross the placenta. This natural mechanism provides an infant with a mixture of many maternal antibodies that can protect it for the first few critical months outside the womb, while its own immune system is gradually developing active immunity. Depending upon the microbe, passive protection lasts anywhere from a few months to a year, but it eventually wanes. Most childhood vaccinations are timed so that there is no lapse in protection against common childhood infections.
Another source of natural passive immunity comes to the baby by way of breast milk (Clinical Connections). Although the human infant acquires most natural passive immunity in utero and some through nursing, breast milk provides IgA antibodies that react against microbes entering the intestine. This unique protection is not available from transplacental antibodies.
Artificial Passive Immunity: Immunotherapy
In immunotherapy, a patient at risk for acquiring a particular infection is administered a preparation that contains specific antibodies against that infectious agent. Pooled human serum from donor blood (gamma globulin) and immune serum globulins containing high quantities of antibodies are the usual sources. Immune serum globulins are used to protect people who have been exposed to certain diseases such as hepatitis A, rabies, and tetanus.
Artificial Active Immunity: Vaccination
The term vaccination originated from the Latin word vacca (cow), because the cowpox virus was used in the first preparation for active immunization against smallpox. Vaccination exposes a person to a specially prepared microbial (antigenic) stimulus, which then triggers the immune system to produce antibodies and memory cells to protect the person upon future exposure to that microbe. As with natural active immunity, the degree and length of protection vary. Commercial vaccines are currently available for many diseases.
