Isolation of a biologically active macrophage receptor for the third component of complement

C3b, the major cleavage fragment of the third component of complement (C3), has been demonstrated to bind to a specific receptor on various mammalian cells¹. Tissue-bound C3b receptors have also been demonstrated, most conclusively in renal glomeruli². On interaction with C3b, C3b receptors are thought to initiate cellular functions such as phagocytosis and to determine the fate of complement-fixing soluble and particulate immune complexes. We report here the isolation by affinity chromatography of a macrophage glycoprotein with an apparent molecular weight (MW) of ∼64,000 that has properties expected of the C3b receptor. It is a cell-surface macromolecule (labelled with ¹²⁵I and lactoperoxidase) which, in its isolated state, retains the ability to bind both C3 and C3b.