<P> 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 not have immunity against . </P> <P> Maternal passive immunity is a type of naturally acquired passive immunity, and refers to antibody - mediated immunity conveyed to a fetus or infant by its mother . Naturally acquired passive immunity can be provided during pregnancy, and through breastfeeding . In humans, maternal antibodies (MatAb) are passed through the placenta to the fetus by an FcRn receptor on placental cells . This occurs predominately during the third trimester of pregnancy, and thus is often reduced in babies born prematurely . Immunoglobulin G (IgG) is the only antibody isotype that can pass through the human placenta, and is the most common antibody of the five types of antibodies found in the body . IgG antibodies protects against bacterial and viral infections in fetuses . Immunization is often required shortly following birth to prevent diseases in newborns such as tuberculosis, hepatitis B, polio, and pertussis, however, maternal IgG can inhibit the induction of protective vaccine responses throughout the first year of life . This effect is usually overcome by secondary responses to booster immunization . </P>

An example of artificially acquired passive immunity is