What is the role of C3b opsonization in bacterial clearance?

• A. It enhances phagocytosis by binding to complement receptors on phagocytes
• B. It directly lyses bacteria via the membrane attack complex
• C. It neutralizes toxins released by bacteria
• D. It inhibits phagocytosis to prevent tissue damage

Answer: (A)

C3b tagging of bacteria enhances their uptake by phagocytes via complement receptors. When the complement system is activated, C3 is cleaved and C3b binds covalently to bacterial surfaces. Phagocytes, such as macrophages and neutrophils, express receptors like CR1 (CD35) and CR3 that recognize C3b (and its derivative iC3b). This binding greatly increases the efficiency of phagocytosis, a process known as opsonization, leading to rapid bacterial clearance. It’s important to note that direct bacterial lysis is accomplished by the membrane attack complex (C5b-9), not by C3b opsonization.