Passive Immunity

Active immunity occurs when an individual is infected with a pathogen or if they are vaccinated. Exposure to the pathogen's antigens by either of these will result in a primary immune response and immunologic memory. However, it is also possible in some circumstances to protect a susceptible person by giving them the antibodies produced by another person. For example, if we were to take serum from people who had previously been infected with hepatitis A virus (HAV), it would contain significant concentrations of IgG against HAV. It is possible to pool serum from previously infected individuals and then inject this immunoglobulin G into individuals who may have been recently been exposed to HAV in order to thwart the infection and prevent them from becoming a clinically active case.

In essence, passive immunization:gives antibodies made by others (e.g., pooled gamma globulin that will immediately recognize and neutralize an antigen to provide immediate protection. However, this passive form of protection bypasses the steps in primary exposure, and it does not produce immunologic memory. Moreover, the protection afforded by this passive form of immunity only lasts as long as the exogenous antibodies, about 3-4 months. After the exogenous antibodies disappear, the individual is just as susceptible as a person who had never been exposed.

CDC Recommendation for Post-Exposure Prophylaxis to Hepatitis A

"Persons who have recently been exposed to HAV and who have not been vaccinated previously should be administered a single dose of single-antigen Hepatitis A vaccine or IG (0.02 mL/kg) as soon as possible, within 2 weeks after exposure. The guidelines vary by age and health status:

  • For healthy persons aged 12 months–40 years, single-antigen Hepatitis A vaccine at the age-appropriate dose is preferred to IG because of the vaccine's advantages, including long-term protection and ease of administration, as well as the equivalent efficacy of vaccine to IG.
  • For persons aged 40 years and older, IG is preferred because of the absence of information regarding vaccine performance in this age group and because of the more severe manifestations of Hepatitis A in older adults. The magnitude of the risk of HAV transmission from the exposure should be considered in decisions to use vaccine or IG in this age group. 
  • Vaccine can be used if IG cannot be obtained.
  • IG should be used for children aged less than12 months, immunocompromised persons, persons with chronic liver disease, and persons who are allergic to the vaccine or a vaccine component"

IgG is able to cross the placenta from mother to fetus. As a result, newborn infants receive some passive immunity from antigens to which their mother has been exposed. However, this passive protection disappears over a period of 3-4 months, so it is important for the infant to develop active immunity through vaccinations (or by being infected and developing clinical disease). The decline in passive immunity in an infant is what dictates the recommended schedule of immunizations for infants.

 


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