In the complement system, C3b functions primarily as which?

In the complement system, C3b functions primarily as which?

• A. Opsonin
• B. Anaphylatoxin
• C. Initiator of the membrane attack complex
• D. Cofactor for factor B

Answer: (A)

Opsonization is the main way the immune system uses C3b. When C3 is activated, C3b covalently binds to the surface of microbes. This tagging is recognized by receptors on phagocytes, such as CR1 (CD35) and CR3, which bind C3b and dramatically enhance uptake and digestion of the pathogen. In this way C3b bridges innate and adaptive immunity by promoting efficient phagocytosis. C3a and C5a are the inflammatory molecules known as anaphylatoxins, so C3b isn’t an anaphylatoxin. The membrane attack complex is initiated by C5b, not by C3b, and while C3b can help amplify the cascade that leads to MAC formation, its primary role isn’t initiating MAC. Regarding factor B, C3b binds to factor B to form the C3 convertase, but it isn’t a cofactor for factor B; rather, factor B is a substrate that is cleaved to form the active enzyme in the presence of C3b.

Opsonization is the main way the immune system uses C3b. When C3 is activated, C3b covalently binds to the surface of microbes. This tagging is recognized by receptors on phagocytes, such as CR1 (CD35) and CR3, which bind C3b and dramatically enhance uptake and digestion of the pathogen. In this way C3b bridges innate and adaptive immunity by promoting efficient phagocytosis.

C3a and C5a are the inflammatory molecules known as anaphylatoxins, so C3b isn’t an anaphylatoxin. The membrane attack complex is initiated by C5b, not by C3b, and while C3b can help amplify the cascade that leads to MAC formation, its primary role isn’t initiating MAC. Regarding factor B, C3b binds to factor B to form the C3 convertase, but it isn’t a cofactor for factor B; rather, factor B is a substrate that is cleaved to form the active enzyme in the presence of C3b.