 Opsonin (source code)

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= Opsonin

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>.<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]{ref}

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.<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]{ref}

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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