PASSIVE IMMUNITY

It is a form of immunity where the individual passively receives immunity produced in another individual. The immunity is received in a "ready-made" fashion. The recipient does not participate in passive immunity. Passive immunity is short lived, inferior to active immunity and have no immunological memory. Immunity can be passively transferred to another individual by humoral or cellular transfer.

Humoral transfer of immunity:  
The immunity in the form of antibodies can be transferred from one individual to another. There is no antigenic stimulus, instead pre-formed antibodies are transferred. Since the recipient immune system doesn't participate, there is no immunological memory. There is no latent period or negative phase and the immunity provided is immediate. Since the antibodies are catabolised and eliminated, the protection offered is usually short term. Frequent transfer of immunoglobulins may cause hypersensitivity reactions and immune elimination. Immune elimination is the removal of transferred immunoglobulin from circulation by binding to antibody against it, which in turn was formed during previous transfer.

There are two types of humoral transfer of immunity: Natural and artificial.
Natural: 
It is the resistance passively transferred from the mother to the baby either through placental transfer or milk (colostrum). Human colostrum is rich in IgA and resistant to intestinal digestion. The transfer of antibodies across the placenta is an active process; hence sometimes the fetal blood may have higher concentration of antibodies than mother's blood. Such passively transferred antibodies protect the infant from common infections for a period of three months. By active immunization of mother during pregnancy, good levels of protective antibody levels can be achieved in the infant. E.g. Administration of tetanus toxoid during pregnancy to avoid neonatal tetanus.

Artificial:
Artificial passive immunity is the transfer of immunity by administration of pre-formed antibodies. The agents used for this purpose include hyperimmune sera of human or animal origin, convalsescent sera, pooled human gamma globulin and monoclonal antibodies.

• Hyperimmune serum is obtained by hyperimmunization of animals (horse). Following repeated immunizations, the animal is bled and antibody is purified and standardized for clinical use. Since antibody obtained from animal origin is foreign in nature, they are removed quickly and also carry the risk of hypersensitivity.

• Convalescent sera are obtained from patients with high levels of specific antibody who are convalescing from infectious disease. Convalescent serum was used to passive immunization against rubella and measles. There is a risk of transmission of blood borne viral diseases.

• Pooled serum is obtained by pooling sera from large number of adults in a community. Since adults would have been exposed to locally prevalent diseases, their serum would contain antibodies against several pathogens. The antibody fraction from such pooled sera is called pooled gamma globulin and is used for passive prophylaxis against common infectious diseases.

• Passive transfer of highly specific monoclonal antibodies against certain viral antigens. Monoclonal antibodies can be modified in a number of ways to make them more protective and less immunogenic to the human recipient.

Passive immunization can be used for immunosuppression as well. Examples include administration of anti-lymphocytic serum in recipients of grafts to prevent graft rejection and administration of Rh immune globulin to suppress the production of antibodies against Rh antigen.

Combined immunization is the method of providing both active and passive immunity. Here the serum is injected on one arm to provide immediate protection and vaccine administered on the other arm to generate active immunity. This method is employed in tetanus, rabies and Hepatitis B exposure.

Examples of artificial passive immunity:

Antibody	Source	Indication
Botulinum antitoxin	Equine	Prophylaxis of botulism
Diphtheria antitoxin	Equine	Treatment of diphtheria
Cytomegalovirus Immune Globulin	Human	Prophylaxis against CMV in bone marrow and kidney transplant recipients
Immune globulin	Human pooled	Pre and post exposure prophylaxis of hepatitis A, post exposure prophylaxis of measles
Hepatitis B Immune Globulin (HBIG)	Human	Pre and post exposure prophylaxis of Hepatitis B
Rabies Immune Globulin (HRIG)	Human	Rabies post-exposure in unimmunized persons
Tetanus Immune Globulin (TIG)	Human	Treatment and prophylaxis of tetanus
Vaccinia Immune Globulin	Human	Treatment of eczema vaccinatum, vaccinia necrosum and occular vaccinia
Varicella zoster Immune Globulin	Human	Prophylaxis of susceptible immunocompromised people against chicken pox
Rh Immune Globulin	Human	Prevention of hemolytic disease of newborn
Antivenom	Equine	Treatment of snake bites and scorpion stings
Polyvalent Immune Globulin	Human	Replacement therapy in B cell deficiencies

Cellular transfer of immunity:
Adoptive immunity is a special kind of passive immunity where protection is gained through the transfer of immune (immunologically competent) cells from one individual to another. The transferred immunocompetent cells are capable of producing antibody or reacting directly and specifically with an antigen. They are derived from lymph node or spleen.

Mechanisms of adoptive transfer of immunity:

• Natural transfer of immune cells: Macrophages, granulocytes, T cells and B cells can be adoptively transferred to breast-fed infants through milk and colostrum.

• Artificial transfer of immune cells: Immunodeficiencies can sometimes be corrected by thymus grafts or bone marrow transplantations.

• Extract of lymphocytes known as "transfer factor" can be used to transfer cell mediated immunity.