What is the primary function of the complement system?

• A. Directly killing pathogens by phagocytosis only
• B. Opsonization, recruitment of inflammatory cells, and lysis of pathogens; enhances innate and adaptive immunity
• C. Producing antibodies
• D. Inhibiting inflammation

Answer: (B)

The primary function of the complement system is to enhance defense by tagging pathogens for attack, signaling and recruiting inflammatory cells, and directly lysing some pathogens. When activated, complement proteins opsonize microbes with C3b, making them easier for phagocytes to engulf. They also generate small peptides like C3a and C5a that attract neutrophils and other immune cells and promote inflammation, helping to contain infection. In addition, the membrane attack complex can disrupt the membranes of certain pathogens, leading to their lysis. This system also supports adaptive immunity by aiding antibody responses and improving antigen presentation. The other options don’t fit: one suggests only phagocytosis as the mechanism, another points to antibody production (a separate B cell function), and another implies inflammation inhibition (which is contrary to how complement often promotes inflammation).