The system of the animal body which protects it from various infectious pathogens is known as immune system. The study of this system is known as immunology.
Active acquired immunity:
The weathering germs or dead pathogen; (antigens) are injected into a normal person to develop immunity by producing antibodies is called Vaccination. These pathogens given in a vaccine, are unable to cause the disease, but are sufficient to stimulate the formation of antibodies by the cells of the host.
This is known as active acquired immunity. The solution of weakened antigens (pathogens) that are injected into the body is called as vaccines. The disease like small pox, polio, typhoid, cholera and plague are treated by active acquired immunity.
Passive acquired immunity:
Sometimes persons do not produce antibodies for some antigens (pathogens). Another animal commonly a horse is injected with the antigen which produce antibodies. These antibodies are isolated and are then injected into the human. This is known as passive acquired immunity. By this process, diseases like diphtheria, scarlet fever and tetanus are treated.
When the maternal antibody, IgG passes through the placenta into the foetus, it develops passive immunity in the infants against infections for the first six months.
After birth, gradually the infants start synthesizing their own antibodies and acquire active immunity.