Which is an example of natural passive immunity?

• A. Vaccination
• B. Administration of monoclonal antibodies or immune globulin
• C. IgG transferred from mother to fetus across the placenta; IgA in breast milk
• D. Long-lived memory cells

Answer: (C)

Natural passive immunity is when the baby receives antibodies from the mother without having to make them itself. The classic example is the transfer of antibodies from mother to fetus across the placenta, and continuing protection after birth from antibodies in breast milk. Specifically, IgG crosses the placenta to the fetus before birth, giving the newborn immediate protection, and IgA in breast milk helps protect the infant’s gut and mucosal surfaces after birth. The infant isn’t actively producing these antibodies at first; they come from the mother, so this is passive and natural.

Vaccination represents active immunity: it stimulates the individual's own immune system to produce antibodies and memory for future protection, and it can be considered artificial active immunity when a vaccine is given. Monoclonal antibodies or immune globulin are externally supplied antibodies given to provide immediate protection, which is passive but artificial, not natural. Long-lived memory cells are part of the active immune response—their development requires exposure to an antigen and represents the body's own ongoing adaptive immunity, not a passive transfer.