Which of the following illustrates passive immunity?

• A. Recovery from hepatitis A infection
• B. Transfer of tumor-infiltrating lymphocytes to a cancer patient
• C. Passage of IgG antibodies through the placenta of a pregnant woman to her fetus
• D. Development of high antibody titers after hepatitis B vaccine

Answer: (C)

Passive immunity is protection conferred to an individual by factors produced by another, providing immediate defense without the recipient activating its own immune response. The placenta transferring IgG antibodies from mother to fetus fits this perfectly: maternal IgG crosses the placental barrier and circulates in the fetus, offering immediate, albeit temporary, protection as the antibodies wane over time. This is different from responses like recovering from an infection or responding to a vaccine, where the individual's own immune system is stimulated to produce antibodies and memory for long-lasting protection. The transfer of tumor-infiltrating lymphocytes is a therapeutic cell transfer, not the classic antibody-based passive immunity, and a vaccine-induced rise in antibodies reflects the host generating its own immune response rather than receiving antibodies directly.