Saturday, 3 November 2012

Elements of Immunity: Acquired Immunity

The resistance developed by man during his life is known as Acquired immunity or Adaptive immunity.
This is distinct from innate immunity in that it is due to specific antibodies or sensitized lymphocytes produces in response to specific antigens. Hence, this immunity is also known as specific immunity.
This acquired specific immunity is of two types namely Active and Passive.
Both active and passive immunity may be Natural or Artificial.
Active immunity is the resistance developed by an individual in response to an antigenic stimulus.

The antigen may gain entrance either by natural infection or through any other sources such as artificial immunization by vaccination.

a) Natural Active Immunity:smallpox
Immunity is developed by the host in response to the antigen that enters by natural infections.
For example, a person attacked by measles or smallpox develops natural active immunity as he recovers from the diseases.
The immunity acquired by way of such infections is also long lasting in many cases. For example, life time immunity is got following certain viral infections such as smallpox, measles and mumps.

b) Artificial Active Immunity:
In artificial active immunity, immunity is attained by the host in response to the antigen got by vaccination. Vaccines are preparations of live (attenuated) or killed microorganism or their products (toxoids).
Live Vaccine (attenuated): in this preparation, live microorganisms are attenuated by different methods. Attenuation results in the loss of pathogenicity without the loss of antigenicity of the microorganisms.
                      E.g.: Anthrax vaccine,
                               BCG (Bacille Calmette Guerin),
                               Sabin Vaccine (oral Poliomyelities Vaccine),
                               Measels Vaccine.
Vaccines prepared with killed microorganisms: In this vaccines, microorganisms are killed in their virulent phase either by heat or antiseptics. While killing, care is taken not to denature the antigen by excessive heat or strong detergents.
                   E.g.: Salk vaccine,
                             Influenza vaccine, vaccine
                             TAB vaccine,
                             Pertussis vaccine,
                             Tetanus Toxoid (TT), 
                             Diptheria Toxoid, 
                            Triple Vaccine (DPT).

Immunization schedule for Children
From 3rd day after birth
3 months
DPT (1) OPV (1)
4 months
DPT (2) OPV (2)
5 months
DPT (3) OPV (3)
10 months
16 months
Booster dose or DPT and OPV

DPT = Diptheria, Pertussis, Tetanus vaccine (Triple vaccine)
OPV = Oral polio vaccine
The immunity that non-immune individual acquires by receiving antibodies or sensitized white blood cells from another immune individual is known as passive immunity.
The immunity is caused by passive immunization is less effective and inferior than that caused by active immunization. The main advantage of passive immunization is that it is immediate in its action of producing immunization and so this method would be used when immediate immunity is needed.
Passive immunity is also two types namely Natural and artificial.

a) Natural Passive immunity:
The immunity transferred from the mother to the child passively is known as natural passive immunity.
  • In human beings, this natural passive immunity occurs mainly by the passage of antibodies from the mother to her unborn child through the placenta during the later part of pregnancy.
  • The antibodies that are transferred are entirely Ig.G as other immunoglobulin sub-types (A, D, E and M) do not pass the placental barrier.
  • In other primates and in most of the other mammals such as pig the transfer of antibodies from the mother to the young one occurs mainly orally through the colostrums (the first milk produced by the mother after the birth of the offspring).
b) Artificial Passive Immunity:

Transfer of immunity from an immunized donor to a non immune recipient by transferring antibodies or immunized lymphocytes is known as artificial passive immunity.
Artificially passive immunity is therapeutically (science of treating diseases) used in the treatment of tetanus, diphtheria, gas gangrene, snake bite and immuno deficiency states.

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