Which is an example of natural active immunity?

• A. Recovery from a natural infection.
• B. Vaccination with a killed vaccine.
• C. Transfer of immune serum.
• D. Exposure to toxins.

Answer: (A)

Natural active immunity occurs when the body generates its own immune response after encountering a pathogen in the natural course of life. After recovering from a natural infection, the immune system creates specific antibodies and memory cells, which persist long-term so future encounters are fought faster and more effectively. This is why recovery from a natural infection is the best example: it relies on the body's own defenses and leaves lasting immunological memory without medical intervention.

Vaccination with a killed vaccine is artificial active immunity because it elicits the immune response through a man-made antigen. Transfer of immune serum provides antibodies from another individual or animal, which is passive immunity and does not involve the body producing its own memory. Exposure to toxins does not reliably produce lasting, pathogen-specific memory, so it’s not an example of natural active immunity.