An opsonin is a substance that coats a target and promotes recognition and uptake by a phagocyte. The name describes a biological role: IgG and complement system fragments such as C3b and iC3b can supply different molecular handles for phagocytosis.[ref]
Receptors for C3b and C3bi promote phagocytosis but not the release of toxic oxygen from human phagocytes by S. D. Wright and S. C. Silverstein. Introduction: recognition of coated particles by complement and Fc receptors
A 1983 experiment separated uptake from another response. Cultured monocytes in contact with appropriate fibronectin-containing surfaces could ingest complement-coated targets without the substantial hydrogen peroxide release seen in the IgG comparison.[ref]
Receptors for C3b and C3bi promote phagocytosis but not the release of toxic oxygen from human phagocytes by S. D. Wright and S. C. Silverstein. Abstract and Figure 1: uptake and oxidative-response comparisons
Thus, coating, receptor engagement, phagocytosis and oxidant production are distinguishable steps. Calling a protein an opsonin does not imply that it digests the target itself, or that every receptor-triggered response accompanies uptake. A useful assay measures binding and internalization separately: a particle attached to a cell membrane is not necessarily inside the biological cell.
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