Which innate defense involves destruction of abnormal cells by NK cells as part of immune surveillance?

• A. Inflammation
• B. Fever
• C. Immune surveillance
• D. Phagocytes

Answer: (C)

The main idea is immune surveillance by natural killer (NK) cells, an innate defense that continuously monitors the body for abnormal cells. NK cells can detect virally infected or transformed cells that often have reduced MHC class I expression or present stress signals. When they recognize such targets, they release cytotoxic granules containing perforin, which creates pores, and granzymes, which induce apoptosis in the abnormal cell. This targeted destruction happens without prior exposure to the threat, fitting the concept of immune surveillance.

Inflammation is a local tissue response to injury or infection, fever is a systemic rise in body temperature, and phagocytes are cells that engulf and digest pathogens and debris. While these are important innate defenses, they do not specifically describe the NK-cell–mediated destruction of abnormal cells as immune surveillance does.