Which of the following is part of the body's defense levels?

• A. Physical barrier
• B. White blood cells
• C. Immune system
• D. Antibodies

Answer: (C)

Defenses occur in layers. The first line blocks entry with physical barriers like skin and mucous membranes. If a pathogen gets through, the immune system coordinates responses using white blood cells to attack invaders and antibodies to target them specifically. The immune system is the overall network that includes both these cellular and molecular components and integrates them with the barriers to defend the body. Therefore, it best describes the body's defense levels since it encompasses all the defense strategies, not just a single element. The other options describe parts of the defense—physical barriers and individual components—but they don’t capture the whole defense system.