In addition to normal innate immunity, every individual develops resistance to certain infectious agents following exposure to them.
Since the immunity is acquired by the individual following exposure to that particular pathogen, it is referred as acquired immunity. It is not natural, but adopted by the individual, and is very specific for the pathogen to which he has been exposed.
Hence this kind of immunity is also referred as adoptive immunity or specific immunity.
TYPES OF ACQUIRED IMMUNITY:
Acquired immunity for specific pathogen can be developed through natural way by getting infection or artificially through vaccination.
Based on the way of acquiring the immunity and the role of immune system, acquired immunity can be classified in to four types. They are Natural acquired active immunity, Natural acquired passive immunity, Artificial acquired active immunity and Artificial acquired passive immunity.
1. Natural Acquired Active Immunity:
Following pathogens /antigens entry in to the body by natural processes (as infection) immune system of the host body produces antibodies and cytotoxic cells to get rid of the pathogens.
Subsequent to recovery some of the antibody producing plasma cells and cytotoxic cells are retained as memory cells.
Memory cells provide immunity to the same pathogen in later period. The acquired immunity may be life-long as with smallpox, measles, chickenpox, yellow fever etc. or may be lost after few years e.g. diphtheria, tetanus or even for lesser period e.g. influenza, pneumonia etc.
2. Natural Acquired Passive Immunity
If antibodies produced by an individual (called donor) in response to a pathogen are naturally transferred to other individual (called recipient), the latter develops immunity. In this process recipients acquire immunity without the involvement of their own immune system.
Since the recipient's immune system is not involved in this process and transfer of antibodies taking place quite naturally it is referred as "natural acquired passive immunity".
Immunity acquired by babies through natural transfer of antibodies from mother via placenta and breast milk is the best example for this type of immunity. These maternal antibodies remain with the child for about three to six months or sometimes twelve to fifteen months.
The maternal antibodies generally provide resistance against whooping cough, diphtheria, German measles, disease of respiratory and gastrointestinal tract, etc.
Artificial Acquired Active Immunity:
Vaccination also induces immunity. The pathogens or parts of the pathogens present in the vaccine induce immunity for that specific pathogen in vaccinated persons. Even though the exposure is not natural, immune system responds to the pathogen administered into the body and develops immunity.
Since the vaccinated individual's immune system is involved in the development of immunity it is referred as "artificial acquired active immunity".
Immunity acquired through vaccines for various infectious diseases such as cholera, tuberculosis, plague, pneumonia, smallpox, polio, tetanus, influenza, measles, rabies, yellow fever etc. belongs to this kind of immunity only.
Artificial Acquired Passive Immunity:
Immunity acquired through direct injection of antibodies and sensitized cells collected from donors is known as "artificial acquired passive immunity". Since recipient's immune system is not involved in the production of antibodies and sensitized cells, it remains for a short period.
In the absence memory cells, immunity developed by this method is purely temporary (only for two to three weeks). Earlier this kind of immunity was an important therapeutic device for disease treatment.
Still it is under use for various viral diseases such as hepatitis B, chicken pox, arthropod-borne encephalitis, and bacterial disease such as diphtheria, tetanus, staphylococcal-poisoning etc. As the above mentioned diseases are very dangerous and fatal, injection of antibodies present in pooled serum into the blood of the susceptible individual saves the life.
The immune fractions and cells can be collected from hyper immune serum of animals or human or from convalescent serum or from pooled sera of healthy individuals.
I. Convalescent serum: The serum of the blood of one or more patients recovering from infection.
II. Hyper immune serum: An antiserum possessing very large quantities of specific antibodies. It can be raised by repeated administration of a series of attenuated or live pathogens into the volunteers or animals. In earlier days antibodies for treatment of tetanus, diphtheria, gas gangrene, snake bite, etc. are produced in this way.
Since there is a possibility of rejection reaction for the serum raised in other animals, now a day's treatment with hyper immune serum is replaced by the antibodies raised through recombinant DNA technology.
Recombinent DNA technology: Joining together of DNA molecules from two different species that are inserted into a host organism to produce new genetic combinations that are of value to science, medicine, agriculture, and industry.
Adoptive immunity is not independent of innate immunity, in fact it is an extend reaction of immune system. Since the rate of multiplication of pathogens that entered into the body is very high, nonspecific innate immunity may not be effective.
Host has to generate specific mechanism to get rid of the pathogens. As re attack by the same pathogen is possible, memory of that pathogen is necessary for a faster action. In addition to this, tremendous pressure imposed by the pathogens with their enormous ability to undergo mutations and molecular heterogeneity insisted evolution of acquired immunity in the higher vertebrates.
The phagocyte cells and the chemicals substances released from pathogens and damaged cells are intimately involved in initiation of specific/ acquired immune reactions.
Hence, both innate and acquired immunity work together to get rid of the foreign agents and the total work is regulated carefully.