Which statement about the complement system is true?

• A. It is a single enzyme
• B. It is a group of at least 9 plasma proteins
• C. It has no role in immune defense
• D. It is produced only in the liver

Answer: (B)

The complement system is a group of multiple plasma proteins that circulate in inactive form and become activated in a cascade to help defend the body against pathogens. The statement that best describes it is that it is a group of at least nine plasma proteins. These components (and their regulators) cooperate through classical, alternative, and lectin pathways to promote opsonization, recruit inflammatory cells, and form the membrane attack complex that can lyse target cells. It’s not a single enzyme, and while most components are produced by the liver, some factors can be produced by other cells too, so it’s not accurate to say it’s produced only in the liver. It also has a well-established role in immune defense, so the claim that it has no role is incorrect.