The reaction of radiolabeled C3b-binding proteins with C3b-coated particles has been investigated. CR1 binding was inhibited by factor H and factor B (in the presence of properdin), but not by properdin alone. CR2 and MCP binding were also inhibited by factor H. Therefore factor H, factor B, CR1, CR2 and MCP probably comprise a group of mutually competitive proteins with similar or overlapping binding sites on C3b. These results correlate with their structural homology and suggest that they all evolved from a single C3b-binding molecule. Factor H, CR1 and MCP are also cofactors for the factor-I-mediated cleavage of C3b. A species incompatibility between rat factor I and human CR1 for the cleavage of human C3b suggests the possibility that cofactors may also function by interacting directly with factor I.

Original languageEnglish
Pages (from-to)30-41
Number of pages12
JournalComplement and Inflammation
Issue number1
StatePublished - 1990


  • C3b/C4b receptor or complement receptor type 1
  • C3b/C4b-binding proteins
  • C3d receptor or complement receptor type 2
  • Factor B
  • Factor H
  • Factor I
  • Formerly gp45-70
  • Membrane cofactor protein
  • Properdin


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