The resistance an individual acquires during life is called acquired immunity. OR Acquired immunity:- Immunity acquired by infection or vaccination (active immunity) or by the transfer of antibody or lymphocytes from an immune donor (passive immunity).
Active Immunity (Adaptive Immunity)
Active immunity or adaptive immunity is capable of recognizing and selectively eliminating specific foreign microorganisms and molecules, i.e. tumor antigens, transplanted antigens, etc. This involves the active functioning of the individual’s immune apparatus, either in producing antibody or creating immune-competent cells for cell-mediated immunity (CMI). Active immunity sets in only after a latent period, which the immunological machinery needs for its functioning. Once developed the active immunity is long lasting. When the individual is facing the same antigen subsequently, there is no latent or lag phase and the immune response is prompt, powerful and prolonged.
Adaptive immunity focuses on four important characteristic features. They are:-
- Antigenic specificity.
- Immunological memory.
- Self/non-self recognition.
The antigenic specificity of the immune system permits it to distinguish minor difference among antigens. The antibodies can distinguish between two protein molecules that differ in only a single amino acid. The immune system is capable of generating tremendous diversity in its recognition molecules, permitting to recognize vast arrays of unique structures on foreign antigens. Once the immune system recognized and responded to an antigen, it exhibits immunological memory to recognize the same antigen, subsequently and react in a heightened manner. Finally, the immune system, normally responds to foreign antigens, indicating that it is capable of distinguishing self from non-self.
The adaptive immunity is not independent of innate immunity. They interact constantly. The phagocytic cells crucial to nonspecific immune responses are intimately involved in igniting the specific immune response. Conversely various soluble factors produced during specific immune response have been shown to augment the activity of these phagocytic cells.
Naturally acquired active immunity
This type of immunity is obtained when a person is exposed to antigens in the course of daily life. Once acquired, the immunity lasts for rest of its life such as in measles and chickenpox. For other diseases, especially in intestinal diseases, the immunity is short lasting. Subclinical infections can also conform immunity as that occurs in tuberculosis. Adults have natural immunity against polio after repeated subclinical infections.
Artificially acquired active immunity
This type of immunity results from vaccination or immunization. Vaccinations may be inactivated bacterial toxins (toxoids), killed microorganisms, live but attenuated microorganisms or parts of microorganisms such as capsules. These substances can no longer cause disease, but can stimulate immune response.
Live vaccines initiate an infection without causing any injury or diseases. The immunity lasts for several years. Booster dose may or may not be required. Live oral (polio) or nasal spray (influenza) vaccines provide local immunity. Killed vaccines are generally less immunogenic than the live vaccines and the immunity lasts only for a short period. Therefore, they are administered repeatedly. Killed vaccines are given parenterally.
Passive immunity is resistance exhibited by the host, when ready-made antibodies or defensive cells are introduced into the body. This form of protection is passive, because the individuals own immune system does not make antibodies or defensive cells against the disease producing agents or toxins.
Naturally acquired passive immunity
This type of immunity involves natural transfer of antibodies from mother to her infant and also from mother to fetus. Certain antibodies (IgA) are passed from the mother to her nursing infants in breast milk, especially in the first secretion called colostrum. The immunity in infants last as long as baby feeds on breast milk.
During pregnancy, some of the maternal antibodies are also transferred through placenta to the fetus. If the mother is immune to diphtheria, rubella or polio, the newborn will be temporarily immune to these diseases as well.
Artificially acquired passive immunity
This type of immunity involves the introduction of antibodies into the body. These antibodies come from animal or person, who is already immune to the disease. They are:-
- Hyperimmune sera of animal or human origin. Common examples are antitetanus serum (ATS), antidiphtheria serum (ADS), anti-gas-gangrene serum (AGS), antisnake venom, etc.
- Convalescent sera from patients very recently recovered from measles, rubella, etc.
- Pooled gamma globulin serum against common infectious diseases.
- Human gamma globulin is also used in the treatment of immunodeficiency diseases.
Indication of Passive Immunization
- For providing immediate and temporary
protection in a non-immune host.
- Treatment of some infections.
- Antilymphocytic serum (ALS) may be given for suppression of lymphocytes in transplantation surgery.
- Passive immunization may also be employed to suppress active immunity, when the latter may be injurious. The commonest example is the use of Rh immunoglobulin during delivery to prevent immune response to rhesus factor in Rhnegative women with Rh-positive babies.
- Combined immunization.
At times, both active and passive immunization is given together. Ideally, it is employed to provide immediate protection to non-immune individual with a tetanus-prone wound. Tetanus immunoglobulin (TIG) will provide immediate passive immunity and toxoid will initiate active immunity.
Adoptive immunity is a special type of immunization, where the immunocompetent cells are injected. At times, instead of whole lymphocytes, an extract of lymphocytes (transfer factor of Lawrence) may be introduced as a therapeutic procedure in certain disease, such as lepromatous leprosy, immunodeficiency diseases such as Wiskott-Aldrich syndrome, disseminated malignancy, etc. Recent studies indicate that the adoptive T cell transfer is done, preferably, in the treatment of virus-related diseases in particular, cytomegalovirus (CMV) infection and Epstein-Barr virus (EBV) infection—associated lymphoproliferative diseases (LPDs).