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chapter 35
Molecular Immunology
TABLE 35-3
Proteins o f the Complement System (
c o n tin u e d )
LECTIN PATHWAY
Intitiated by binding of MBP to mannose residues on foreign cell surface molecules
Protein/Subunit(s)
Serum Cone.
Molecular
Weight
Description and Role of the
Active Form
(mg/L)
(chains)
Active Forms in These Pathways
MBP
(mannan-binding
protein)
MASP-1
Protein that recognizes mannose residues on
oligosaccharides on cell surfaces
Homologous toC lr& C ls, cleaves C1, C2 and C4 directly
MASP-2
Homologous to Clr & Cls, cleaves C4 directly
MEMBRANE ATTACK
PATHWAY
Final, common pathway by which the cell is made permeable
Protein/Subunit(s)
Serum Cone.
Molecular
Weight
Description and Role of the
Active Form
(mg/L)
(chains)
Active Forms of These Pathways
C5
55-113
196,000
The membrane attack complex (MAC) is formed by
C5a
_
_
stepwise assembly of C5b through C9.
Anaphylatoxin (potent), has chemotactic activity,
C5b
C6
179-239
125,000
increases vascular permeability and causes mast cell
degranulation
Part of the MAC
Component of MAC, steps involving C5, C6 & C7 occur
C7
27-74
120,000
in solution
Component of MAC
C8
49-106
150,000
Component of MAC on membrane surface
C9
33-95
66,000
Component of MAC on membrane surface
REGULATORY REACTIONS
Protein/Subunit(s)
Serum Cone.
Active Form
(mg/L)
Molecular
Weight
(chains)
Description and Role of the
Active Form in These Pathways
Cl inhibitor
180
109,000
SERPIN, (SERine Protease Inhibitor), Inactivates
C 1
r, C 1
s by covalent bond formation
Anaphylatoxin inactivator
(carboxypeptidase N)
310,000
Protease, Inactivates C3a and C5a anaphylatoxins by
cleavage of X-Arg peptide bond
Factor I
25-50
88,000
Protease, Cleaves C3b that is bound to H or CR1 and
C4b that is bound to C4BP
Factor H
405-717
15,000
Binding protein dissociates C3bBb
C4b binding protein (C4BP)
250
570,000
Binding protein dissociates C4b2a
Decay accelerating factor
70,000
Dissociates C3bBb and Binding protein C4b2a from
host cells
Complement receptor (Type 1)
(Crl)
200,000
Binding protein when bound to C3b or C4b, allows
them to be cleaved by Factor I
Vitronectin (S protein)
500
80,000
Binding protein Binds to MAC
