Specific immunity
Active and passive immunity
There are two basic mechanisms for acquiring immunity - active and passive.
Active immunity is protection that is produced by the person’s own immune system. This type of immunity is usually permanent.
Passive immunity is protection by products produced by an animal or human, and transferred to another human, usually by injection. Passive immunity often provides effective protection, but this protection wanes (disappears) with time, usually a few weeks or months.
Passive immunity is the transfer of antibody produced by one human or other animal to another. Passive immunity provides protection against some infections, but this protection is temporary. The antibodies will degrade during a period of weeks to months and the recipient will no longer be protected. The most common form of passive immunity is that which an infant receives from its mother. Antibodies are transported across the placenta during the last 1-2 months of pregnancy. These antibodies will protect the infant from certain diseases for up to a year. Protection is better against some diseases (e.g., measles, rubella, tetanus) than others (e.g., polio, pertussis).
Active immunity is stimulation of the immune system to produce antigen-specific humoral (antibody) and cellular immunity. Unlike passive immunity, which is temporary, active immunity usually lasts for many years, often for a lifetime. One way to acquire active immunity is to have the natural disease. In general, once persons recover from an infectious disease, they will be immune to those diseases for the rest of their lives. Pertussis is an exception.
The persistence of protection for many years after the infection is known as immunologic memory. Following exposure of the immune system to an antigen, certain cells (memory B-cells) continue to circulate in the blood (and also reside in the bone marrow) for many years. Upon re-exposure to the antigen, these memory cells begin to replicate and produce antibody very rapidly to re-establish protection.
Another way to produce active immunity is by vaccination. Vaccines interact with the immune system and often produce an immune response similar to that produced by the natural infection, but do not subject the recipient to the disease and its potential complications. Vaccines produce immunologic memory similar to that acquired by having the natural disease.
Many factors may influence the immune response to vaccination. These include the presence of maternal antibody, nature and dose of antigen, route of administration, and the presence of adjuvants (e.g., aluminum-containing materials added to improve the immunogenicity of the vaccine). Host factors such as age, nutritional factors, genetics, prolonged psychological stress and coexisting disease, may also affect the response.