Which statement best distinguishes active immunity from passive immunity?

• A. Active immunity is generated by transfer of antibodies from another source.
• B. Passive immunity is generated by the host's own immune response.
• C. Active immunity is generated by the host's own immune response (infection or vaccination); passive immunity is provided by transfer of antibodies from another source.
• D. Passive immunity uses memory from prior exposure.

Answer: (C)

The core idea is how immunity is established: whether the body builds it itself or simply receives it from another source. In active immunity, the body's immune system is stimulated by infection or vaccination, leading to the production of specific antibodies and memory B cells. This memory enables a faster, stronger response if the organism is exposed again, and the protection tends to be long-lasting. In passive immunity, antibodies come from someone else and are given to the person; the recipient’s own immune system isn’t activated to produce those antibodies, so there is no memory formed and the protection is temporary as the borrowed antibodies fade away. The best statement captures this distinction by stating that active immunity is generated by the host’s own immune response (infection or vaccination) and passive immunity is provided by transfer of antibodies from another source. The other options mix up who generates the response or misstate memory as a feature of passive immunity.