Table 1 Alternative,classical and lectin pathway proteins Concentration Protein Structure (ug mL-1) Cellular sources Key function Alternative pathway (AP) Factor B 93kDa 210 Hepatocytes,mononuclear phagocytes, Catalytic subunit of AP C3 convertase,forms epithelial and endothelial cells, part of the C5 convertase adipocytes,fibroblasts Factor D 24kDa 1-2 Mononuclear phagocytes,adipocytes Cleaves factor B that is bound to C3b or C3(H2O) Properdin 55-220kDa 25 Mononuclear phagocytes Stabilizes AP C3 convertase CLOPEDIA monomer to tetramer C3(185 kDa) 110 kDa o chain 1300 Hepatocytes,mononuclear phagocytes, Activated C3(C3b)covalently binds to 75kDaβchain epithelial and endothelial cells, activating surfaces.It forms part of the C3 adipocytes,fibroblasts and C5 convertases.Forms part of both altemative and classical pathways Factor H 150 kDa 500 Hepato phagocytes, elial cells, Accelerates the dissociation of APC3 convertase.Cofactor for factor fibroblasts,B cells,keratinocytes, myoblasts FactorI 88 kDa 必 Hepatocytes,mononuclear phagocytes, C4b/C3b inactivator myoblasts,adipocytes,fibroblasts,Bcells Classical pathway (CP) Clq (462 kDa) Hexamer.Subunit contains:80 Hepatocytes,mononuclear phagocytes, Binds to IgM or IgG or C-reactive protein fibroblasts,gastrointestinal epithelial cells (CRP)and initiates the classical pathway (x6)26.5 kDa B chains (x6)24 kDa C chains w.els.net Cir 83kDa 50 Hepatocytes,mononuclear phagocytes, Cleaves C1s fibroblasts,gastrointestinal epithelial cells Cis 83kDa Hepatocytes,mononuclear phagocytes Cleaves C4 and C2 C4(205kDa) 97 kDa,a chain 600 Hepatocytes,mononuclear phagocytes Activated C4(C4b)covalently binds to 75 kDa,Bchain inary and alveolar Forms part of classical C3 33 kDa,Ychain type lI epithelial cells convertase C2 110kDa Hepatocytes,mononuclear phagocytes, Catalytic subunit of the CP C3 convertase. fibroblasts,genitourinary and alveolar Forms part of the C5 convertase ypeⅡepithelial cells Table 1 Alternative, classical and lectin pathway proteins Protein Structure Concentration (µg mL–1) Cellular sources Key function Alternative pathway (AP) Factor B 93 kDa 210 Hepatocytes, mononuclear phagocytes, epithelial and endothelial cells, adipocytes, fibroblasts Catalytic subunit of AP C3 convertase, forms part of the C5 convertase Factor D 24 kDa 1–2 Mononuclear phagocytes, adipocytes Cleaves factor B that is bound to C3b or C3(H2O) Properdin 55–220 kDa monomer to tetramer 25 Mononuclear phagocytes Stabilizes AP C3 convertase C3 (185 kDa) 110 kDa α chain 75 kDa β chain 1300 Hepatocytes, mononuclear phagocytes, epithelial and endothelial cells, adipocytes, fibroblasts Activated C3 (C3b) covalently binds to activating surfaces. It forms part of the C3 and C5 convertases. Forms part of both alternative and classical pathways Factor H 150 kDa 500 Hepatocytes, mononuclear phagocytes, epithelial and endothelial cells, fibroblasts, B cells, keratinocytes, myoblasts Accelerates the dissociation of AP C3 convertase. Cofactor for factor I Factor I 88 kDa 35 Hepatocytes, mononuclear phagocytes, myoblasts, adipocytes, fibroblasts, B cells C4b/C3b inactivator Classical pathway (CP) C1q (462 kDa) Hexamer. Subunit contains: (( ×1)A+( ×1)B+( ×1)C) (×6) 26.5 kDa A chains (×6) 26.5 kDa B chains (×6) 24 kDa C chains 80 Hepatocytes, mononuclear phagocytes, fibroblasts, gastrointestinal epithelial cells Binds to IgM or IgG or C-reactive protein (CRP) and initiates the classical pathway C1r 83 kDa 50 Hepatocytes, mononuclear phagocytes, fibroblasts, gastrointestinal epithelial cells Cleaves C1s C1s 83 kDa 50 Hepatocytes, mononuclear phagocytes, fibroblasts, gastrointestinal epithelial cells Cleaves C4 and C2 C4 (205 kDa) 97 kDa, α chain 75 kDa, β chain 33 kDa, γ chain 600 Hepatocytes, mononuclear phagocytes, fibroblasts, genitourinary and alveolar type II epithelial cells Activated C4 (C4b) covalently binds to activating surfaces. Forms part of classical C3 convertase C2 110 kDa 20 Hepatocytes, mononuclear phagocytes, fibroblasts, genitourinary and alveolar type II epithelial cells Catalytic subunit of the CP C3 convertase. Forms part of the C5 convertase Complement 4 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net