Which of the following best explains why complement activation does not destroy normal body cells?

• A. Normal body cells express complement regulatory proteins that inhibit complement activation on their surfaces.
• B. Normal body cells lack membrane-bound receptors for complement proteins.
• C. Normal body cells rapidly degrade all complement proteins in their cytoplasm.
• D. Complement proteins are only produced in response to viral infections.

Answer: (A)

Verified step by step guidance

1. Understand the role of the complement system: it is a part of the innate immune system that helps destroy pathogens by forming membrane attack complexes (MAC) that lyse target cells.
2. Recognize that complement activation must be tightly regulated to prevent damage to the body's own cells, as uncontrolled activation could lead to self-destruction.
3. Identify that normal body cells have specific mechanisms to protect themselves, such as expressing complement regulatory proteins on their surfaces, which inhibit the formation or function of complement complexes.
4. Evaluate the incorrect options: normal cells do have receptors that can interact with complement proteins, they do not degrade complement proteins inside the cytoplasm as a primary defense, and complement proteins are constitutively present and not only produced during viral infections.
5. Conclude that the best explanation is that normal body cells express complement regulatory proteins that prevent complement activation on their surfaces, thereby protecting themselves from complement-mediated lysis.