What is the role of complement protein C3b in immune defense?

• A. C3b acts as an opsonin to promote phagocytosis and participates in formation of C3/C5 convertases.
• B. C3b dissolves immune complexes in the plasma.
• C. C3b directly lyses pathogens by forming membrane pores.
• D. C3b inhibits inflammation by blocking cytokines.

Answer: (A)

C3b’s main role is to tag pathogens for destruction and to amplify the complement response. It coats microbial surfaces, serving as an opsonin that is recognized by phagocyte receptors (such as CR1), which enhances phagocytosis. At the same time, C3b combines with other components to form C3/C5 convertases, boosting the cascade: this amplification step creates more C3b and C5 convertases that drive further complement activation and more immune attack on the invader.

It doesn’t dissolve immune complexes by itself, and it doesn’t directly lyse cells by forming pores—that’s the job of the membrane attack complex formed later in the cascade. Nor does C3b suppress inflammation; complement activation actually promotes inflammation through other fragments like C3a and C5a.