Which complement component is known for both opsonization and activation of C5?

C3b is the complement component that serves a dual role in the immune response, particularly in opsonization and the activation of C5. Opsonization is the process by which pathogens are marked for destruction by phagocytic cells, improving their recognition and absorption by immune cells. C3b binds to the surface of pathogens, facilitating their engulfment by macrophages and neutrophils, which have receptors for C3b.

In addition to its role in opsonization, C3b is crucial for the activation of the complement component C5. When C3b binds to C5, it leads to the cleavage of C5 into C5a and C5b, triggering further downstream events in the complement cascade, including the formation of the membrane attack complex and the eliciting of inflammatory responses through C5a.

The other complement components mentioned do not fulfill both of these functions. C2 and C4 are primarily involved in the classical pathway, contributing to the formation of the C3 convertase but not directly in opsonization or activating C5. C5a, while a product of C5 activation, does not participate in opsonization.