= Complement receptor 1
\b[Complement receptor 1] is a membrane <glycoprotein> that recognizes <C3b> and participates in regulation and handling of complement-tagged material. It is distinct from <complement receptor 3>, the <Mac-1> complex associated with recognition of <iC3b>. Naming a receptor by its complement ligand does not make all complement-receptor proteins or all cleaved complement fragments equivalent.<Identification of the membrane glycoprotein that is the C3b receptor of the human erythrocyte, polymorphonuclear leukocyte, B lymphocyte, and monocyte by D. T. Fearon>[JEM 152,20–30(1980), abstract: membrane gp205 and C3b-receptor identification.]{ref}
The originating identification connected a physical membrane component with biochemical activity. <D. T. Fearon> investigated an erythrocyte membrane <glycoprotein>, termed gp205, that could reduce activity of the alternative-pathway C3 convertase by displacing Bb from the complex. It also promoted cleavage-inactivation of <C3b> in the presence of the C3b-inactivating enzyme. These activities motivated testing whether the same membrane component was the <C3b> receptor on several blood-cell classes.<Identification of the membrane glycoprotein that is the C3b receptor of the human erythrocyte, polymorphonuclear leukocyte, B lymphocyte, and monocyte by D. T. Fearon>[Abstract: convertase decay, inactivation cofactor activity and erythrocyte/leukocyte receptor identity.]{ref}
The example distinguishes recognition from catalysis: a membrane <receptor> can bring a substrate into an arrangement that assists an enzyme without itself being that enzyme. Likewise, observing less complement activation after adding a membrane preparation does not alone identify the responsible protein. Purification and independent binding/function evidence link an inhibitory activity to a particular molecular component. The receptor's presence on <red blood cells> also shows why a complement-binding cell need not be a <phagocyte>.
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