I.

COMPLEMENT
 Small soluble proteins that regulate the immune system, orchestrating both innate immunity and the adaptive response to infection.
 Order of Discovery: C1 2 3 4 5 6 7 8 9
 Order of Activation: C1 4 2 3 5 6 7 8 9
Characteristics:
 Soluble and cell bound proteins
 Heat labile proteins (Thermolabile: 56℃ - 30 minutes) and can regain activity at 7-37℃. Activity is lost in 3-4 days during refrigeration and
may deteriorate in 1-2 days at room temperature.
 Predominantly inactive molecules (may be inactivated by acids, alkalis, proteolytic enzymes, ether, chloroform, alcohol, bile salts, soap)
 All produced on liver except C1 components (intestinal epithelial pathway) and Factor D (Adipose tissue)

Classical Pathway Alternative/Properdin Pathway Lectin Pathway

Antibody dependent Yes (Igm and 2 IgG) No No
Stimulus Immune complexes, binds to CRP, E. Lipopolysaccharide, Fungal Cell Mannose and several other sugars
coli, Mycoplasma, Protozoa (Zymosan), Yeast, Parasites found primarily on bacteria, some
(trypanosomes), tumor cell lines, yeasts, viruses, and several parasites
cobra venom factor
Calcium dependent Yes No (Magnesium) Yes
Recognition unit C1qrs C3, Factor B, Factor D lectins, ficolins, and collectins, binding
of MBL (MASPs)
Activation unit (all are C3 and C5 C4b2a (C3 convertase) & C4b2a3b (C5 C3bBb (C3 convertase) & C3bBbPC3b Same sa classical pathway (but C3
convertase) convertase) (C5 convertase) convertase can be activated without
C2 or C4
MAC C56789
End result Cell lysis
Classical Pathway Alternative Pathway
C1q - binds to the Fc portion of IgG (CH2) & IgM (CH3) Factor B - binds to C3b to form C3 convertase
C1r - activates C1s; zymogens Factor D - cleaves Factor B into Bb in the presence of C3 and Mg ions
C1s - cleaves C4 and C2; zymogens Properdin - stabilizes C3 convertase (C3bBb)
C3 - key intermediate in all pathway
C5 - initiates MAC Lectin Pathway
C6 - stabilizes and binds C5b MBL - binds to mannose
C7 - has affinity to lipid components MASP 2 - acts like C1s
C8 - expose hydrophobic region (pore formation) MASP 1,2,3 - binds to form an activated C1 like complex
C9 - accelerates cell lysis through polymerization

C3a, C4a, C5a - anaphylaxis; causes increased vascular permeability, contraction of smooth muscle, and release of histamine from basophils
C3b, C4b, C5b - opsonization (promotes phagocytosis)
C5b6789 - membrane attack complex
C5a - chemotaxin
C2b (Prokinin) - accumulation of body fluid; can cause edema

Pathways:
1. Classical Pathway
 first to be studied
 Initiated by Ag-Ab complex (IgM>>IgG3>>IgG1>>IgG2)

2. Alternative Pathway/Properdin Pathway

 Initiated by: aggregates of IgA, yeast cell wall or zymosan, lipopolysaccharides, cobra venom factor
 Bypasses C1, C4, and C2
 Begins with the activation of C3
 Activated at slow rate by water and plasma enzymes

3. Lectin Pathway
 MBL: Mannose binding lectin - attaches to mannose or similar sugars in cell walls or outer membranes of microorganisms
 Cleaves C4 and C2 (formation of C3 convertase); proceed as classical

PLASMA COMPLEMENT REGULATORS

Action Source
C1 inhibitor inhibits activation at the first stages of both Liver macrophage
the classical and lectin pathways by inhibiting
MASP-2, C1r, C1s
Factor 1 Inhibits C4b and cleaves C3b; acts as a cofactor
with DAF, CR1, MCP
Factor H Prevents the binding of Factor B and C3b; 100x
affinity to C3b
C4-binding protein/membrane cofactor/CD46 abundant in the plasma and is capable of Found on cell membranes of virtually all
combining with either fluid-phase or cell- epithelial and endothelial cells except
bound C4b; Cofactor to C4b, C3b, Factor B erythrocytes
Most efficient cofactor for Factor 1-mediated
cleavage of C3b
CR1/CD35 found mainly on the cell surface of peripheral Blood cells
blood cells; binds with C3b; acts as a cofactor
with Factor 1
Also plays a key role in the clearance of
immune complexes
Decay-accelerating factor/CD55 Binds to C4b, C3b, and dissolves C2a peripheral blood cells, endothelial cells,
(disassociation the C3 convertases of both fibroblasts, and numerous types of epithelial
classical and alternative pathway); prevents cells.
bystander lysis
Membrane inhibitor of reactive lysis Inhibits MAC through binding with C8 to Blood cell
MIRL/CD59 prevent insertion of C9
S protein/vitronectin Soluble; prevent attachment of Cbb67 complex
to cell membrane
 DEFICIENCIES OF COMPLEMENT COMPONENTS
DEFICIENT COMPONENTS ASSOCIATED DISEASES
C1qrs Lupuslike syndrome; recurrent infections
C2 Lupuslike syndrome, recurrent infections, atherosclerosis
Most common complement deficiency
C3 Glomerulonephritis, hemolytic uremic syndrome (atypical type)
Most severe complement deficiency; most commonly measured
C4 Lupuslike syndrome
C5, C8, Properdin Neisseria infection
C9 No known disease association
C1-INH Hereditary angioedema
DAF, MIRL (CD 55 & CD 59) Paroxysmal nocturnal hemoglobinuria
Factor H or I Recurrent pyogenic infections
MBL Pneumococcal diseases, sepsis, neisseria infection
MASP-2 Pneumococcal infections

 Laboratory Detection of Complement Abnormalities

 Immunologic assays of individual components
- Radial immunodiffusion (RID) - not in favor with automation
- Nephelometry
 Assay for the classical pathway
- Hemolytic titration (CH50) assay - measure of total complement activity. Complement needed to lyse 50% of antibody sensitized RBC
(sheep RBC)
 Alternative pathway
- AH50
- ELISA - measures activation fragments