Passive immunization is the act of providing antibodies to the immune system that it did not create itself instead of stimulating the immune system to create antibodies. The two most common forms of immunization are acquiring it from maternal antibodies during pregnancy — when the pregnant individuals’ antibodies cross the placenta to the fetus — and receiving it from a blood product containing the antibody, such as immune globulin.
Both examples can be seen with the two types of respiratory syncytial virus (RSV) immunization. The parental RSV vaccine is given during pregnancy to stimulate the gestating parent’s immune system to create antibodies that then cross the placenta to the fetus. The newborn still has those antibodies to protect it for months after birth. Or, infants can receive the nirsevimab, a monoclonal antibody against RSV that passively immunizes the infant against RSV.