Which complement protein acts as an opsonin in the immune response?

• A. C5a
• B. C1q
• C. C9
• D. C3b

Answer: (D)

Verified step by step guidance

1. Understand the role of opsonins: Opsonins are molecules that enhance phagocytosis by marking antigens for an immune response. They bind to the surface of pathogens and make them more recognizable to phagocytes.
2. Identify the complement system: The complement system is a part of the immune system that enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells, promote inflammation, and attack the pathogen's cell membrane.
3. Learn about complement proteins: Complement proteins are a group of proteins that play a crucial role in the immune response. They are activated in a cascade manner and have various functions, including opsonization, chemotaxis, and cell lysis.
4. Focus on C3b: C3b is a fragment of the complement protein C3. It acts as an opsonin by binding to the surface of pathogens, thereby facilitating their recognition and ingestion by phagocytes.
5. Differentiate between complement proteins: While C5a is involved in chemotaxis and inflammation, and C1q is part of the classical pathway activation, C3b specifically functions as an opsonin, marking pathogens for phagocytosis.