Which complement component acts as an opsonin after activation?

Which complement component acts as an opsonin after activation?

• A. C3a
• B. C3b
• C. C5a
• D. C4b

Answer: (B)

Opsonization is the process of tagging a microbe to enhance its uptake by phagocytes. When the complement system is activated, C3 is split into C3b and C3a. C3b covalently binds to the surface of the pathogen and serves as an opsonin, providing binding sites for phagocyte receptors (such as CR1 and CR3) and dramatically increasing phagocytosis. The other fragments have different roles: C3a and C5a are anaphylatoxins that drive inflammation and recruit immune cells, while C4b is part of the classical pathway C3 convertase (C4b2a) and helps propagate the cascade rather than directly tagging the pathogen for phagocytosis. So, the fragment that acts as the opsonin after activation is C3b.

Opsonization is the process of tagging a microbe to enhance its uptake by phagocytes. When the complement system is activated, C3 is split into C3b and C3a. C3b covalently binds to the surface of the pathogen and serves as an opsonin, providing binding sites for phagocyte receptors (such as CR1 and CR3) and dramatically increasing phagocytosis. The other fragments have different roles: C3a and C5a are anaphylatoxins that drive inflammation and recruit immune cells, while C4b is part of the classical pathway C3 convertase (C4b2a) and helps propagate the cascade rather than directly tagging the pathogen for phagocytosis. So, the fragment that acts as the opsonin after activation is C3b.