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Active and Passive Immunity

Immunity is acquired either actively or passively, depending on weather the immune person produces his or her own protective antibodies or T cells or receives presynthesized antibody or sensitized lymphocytes from another individual. Both active and passive immunity can be acquired either naturally or artificially.
Active Immunity: active immunity is the production of antibodies or specialized lymphocytes by the host as a result of exposure to a foreign antigen. It is characterized by; (1) a latent period of at least 2 weeks between initial exposure to the antigen and development of protective immunity and (2) an extended duration of immunity that often lasts for years.
Natural active immunity develops following exposure to an antigen as a result of natural infection. Immunity is directed against the specific infectious agent as well as its toxic byproducts. Immunologic recall in both cell-mediated and humeral responses maintains protection for months and often for years. The protective efficiency of natural active immunity is demonstrated by the infrequency of second attacks of chicken pox, mumps and measles. Even in the absence of overt symptoms these infections may stimulate lifelong immunity.

Artificial active immunity: It is similar to natural active immunity except for the natural of the antigen and the method of introduction into the host. Instead of natural infection by potential virulent microbes a vaccine is intentionally introduced into the body by a clinical procedure such as injection. A vaccine is antigenically similar to a pathogen or to its toxic byproducts but has been treated so that it can be administered to people with little danger of disease. The vaccine sensitizes the immune system to the corresponding pathogen, inducing immunity without the danger of infectious disease developing during the lag period. In immunized people natural exposure to the corresponding virulent pathogen triggers a protective anamnestic response that eradicates the pathogen or neutralizes toxins before symptoms of disease develop.
Active immunity is carried out by: Vaccines, Vaccines that are killed organisms, Attenuated Organisms, Purified Antigens Fraction and Genetically Engineered Vaccines.
Passive Immunity: antibodies produced by active immunity in one individual can be transferred in serum to a nonimmune recipient. The antibody recipient is then said to have passive immunity. Although these antibodies provide immediate protection, they are eventually depleted and are not replaced by the body. Passive immunization therefore, usually lasting no more than a few weeks. Cell-mediate immunity can be passively transferred by immune lymphocytes derived from a sensitized donor. Furthermore a soluble component called transfer factor can be extracted from immune lymphocytes and used to specially sensitize a nonimmune recipients T cells. Transfer factor has been successfully used to control severe Candida albicans infections in persons with apparently deficient CMI against this opportunistic yeast.
Natural Passive Immunity: It helps protect newborns from infectious disease. The fetus which is incapable of producing antibodies, become passively immunized by maternal antibodies that cross the placenta from the mother to the fetus. Maternal antibodies persist for many weeks following birth and help protect the baby until he or she can actively produce antibodies.
Artificial passive immunity: It is a valuable therapeutic tool when immediate protection is required, with no time to wait for active immunity to develop. Antibodies or immune T lymphocytes produced by one individual are injected into a recipient to provide temporary protection.

“The injured patient will sometime receive two tetanus shots, one a toxoid to boost active immunity for subsequent years of protection, the other an antitoxin preparation to provide immediate protection against the toxin. The two shots are given in different arms so they don’t neutralize each other”.
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