Passive immunity is the transfer of active
humoral immunity in the form of ready-made
antibodies. Passive
immunity can occur naturally, when maternal antibodies are transferred to the
fetus through the
placenta, and it can also be induced artificially, when high levels of antibodies specific to a
pathogen or
toxin (obtained from
humans,
horses, or other
animals) are transferred to non-
immune persons through
blood products that contain antibodies, such as in
immunoglobulin therapy or
antiserum therapy.
Passive immunization is used when there is a high risk of infection and insufficient time for the body to develop its own immune response, or to reduce the symptoms of ongoing or
immunosuppressive diseases. Passive immunization can be provided when people cannot synthesize antibodies, and when they have been exposed to a disease that they do have immunity against.