In animals in addition to physical barries (skin + mucous membranes ) and phagocytes, there is a third mechanism, to defend their bodies against the foreign invader: this the immune system.
Types of immune system(immunity):
The use of vaccines, which stimulate the production of antibodies in the body, and making a person immune against the diease or infections, is called active immunity.But this active immunity has been achieved by artificially introducing, antigens in the body, so it's called artificially induced active immunity.
Help Through Active Immunity;
when a person is exposed to an infection (antigen) - becomes ill, and in most cases survives then this immunity, developed against that diease is called naturally induced immunity or auto immune response.
In constrast to active immunity, in which case antigens are introduced to stimulate the production of antibodies, by artificial or natural method; antibodies are injected in the form of intisera, to make a person immune against a diease, this is called passive immunity.
Help Through Passive Immunity;
The method of passive immunization is use to combat active infections of tetanus, infections hepatitis, rabies,snake bite venom etc.In the case of snake bite venom passive immunity is produced by intitoxins - so the serum is called antivenom serum.Passive immunity respone is immediate, but not long lasting.Because no time is taken for the production of sufficient level of antibodies,(as antibodies are being injected) and after the level of antibodies is reduced or they are used up-no more antibodies production is there.
Dieases of Immune System:
AIDS:(acquired immune deficiency Syndrome) is diease caused by a virus the affected persons suffer from deficiency in their immune system of the body, and the immune collapses thus the aids victim often succumbs to bacterical diease are cancer that under normal circumstances, the immune system can overcome there is no known cure of the diease in can speard by blood transfusion and by sexual contact with the infected persons.