C3b, C4b, and C1q are important complement proteins that mediate opsonization. As a part of the alternative complement pathway, the spontaneous activation of a complement cascade converts C3 to C3b, a component that can serve as an opsonin when bound to an antigen's surface.

What complement protein causes phagocytosis?

Phagocytosis – by opsonizing antigens. C3b has most important opsonizing activity. (Alternative Complement Pathway) Inflammation – by attracting macrophages and neutrophils.

What does a complement protein do when it Opsonized a bacterium?

Opsonization and Membrane Complement Receptors

The complement (C) system plays a major role in opsonization by coating particles such as bacteria with fixed C3 and C4. This leads to binding of the bacteria to phagocyte C3 receptors and clearance of the bacteria.