In vivo, both IgG and C3b are important opsonins. Phagocytic cells, either neutrophil or macrophage, have specific surface receptors for the Fc region of the IgG molecule and C3b. The opsonized microbe is ingested through receptor-mediated phagocytosis.

Are macrophages opsonin?

Macrophages display receptors for the later-acting C3b and C4b components of complement (CR1 receptors), and these components cooperate with the corresponding Fcγ receptors to hasten destruction of the red cell by acting as opsonins.

What functions as an opsonin?

The function of opsonins is to react with bacteria and make them more susceptible to ingestion by phagocytes. Opsonization of bacteria may occur by three different mechanisms. First, specific antibody alone may act as an opsonin.