Complement refers, historically, to fresh serum capable of lysing antibody (Ab)-coated cells. This activity is destroyed (inactivated) by heating serum at 56EC for 30 minutes. The lytic activity of complement is decreased in certain diseases, e.g. SLE, serum sickness, chronic infections, complement deficiencies, etc. complement functions Host benefit: opsonization to enhance phagocytosis phagocyte attraction and activation lysis of bacteria and infected cells regulation of antibody responses clearance of immune complexes clearance of apoptotic cells Host detriment: Inflammation, anaphylaxis

2. Complement: History
Discovered in 1894 by
Bordet
It represents lytic activity
of fresh serum
Its lytic activity destroyed
when heated at 56C
for 30 min

3. Complement functions
• Host benefit:
– opsonization to enhance phagocytosis
– phagocyte attraction and activation
– lysis of bacteria and infected cells
– regulation of antibody responses
– clearance of immune complexes
– clearance of apoptotic cells
• Host detriment:
– Inflammation, anaphylaxis

4. Proteins of the complement
system (nomenclature)
• C1(qrs), C2, C3, C4, C5, C6, C7, C8, C9
• factors B, D, H and I, properdin (P)
• mannose binding lectin (MBL), MBL associated
serine proteases (MASP-1 MASP-2)
• C1 inhibitor (C1-INH, serpin), C4-binding
protein (C4-BP), decay accelerating factor
(DAF), Complement receptor 1 (CR1), protein-
S (vitronectin)

5. • C-activation: alteration of C proteins such that they
interact with the next component
• C-fixation: utilization of C by Ag-Ab complexes
• Hemolytic units (CH50): dilution of serum which
lyses 50% of a standardized suspension of Ab-coated
r.b.c
• C-inactivation: denaturation (usually by heat) of an
early C-component resulting in loss of hemolytic activity
• Convertase/esterase: altered C-protein which acts
as a proteolytic enzyme for another C-component
Definitions

6. Activation product of complement
proteins (nomenclature)
When enzymatically cleaved, the larger moiety,
binds to the activation complex or membrane
and the smaller peptide is released in the
microenvironment
Letter “b” is usually added to the larger,
membrane-binding, peptide and “a” to the
smaller peptide (e.g., C3b/C3a, C4b/C4a,
C5b/C5a), EXCEPT C2 (the larger, membrane-
binding moiety is C2a; the smaller one is C2b)
Activated component are usually over-lined: e.g.
C1qrs

7. Pathways of complement
activation
CLASSICAL
PATHWAY
ALTERNATIVE
PATHWAY
activation
of C5
LYTIC ATTACK
PATHWAY
antibody
dependent
LECTIN
PATHWAY
antibody
independent
Activation of C3 and
generation of C5 convertase

8. Components of the Classical
Pathway
C4
C3
C1 complex

9. Classical Pathway
Generation of C3-convertase

10. Classical Pathway
Generation of C3-convertase
C4b
_____
C4b2a is C3 convertase

11. Classical Pathway
Generation of C5-convertase
C4b
C3b
________
C4b2a3b is C5 convertase;
it leads into the Membrane
Attack Pathway

12. 12
Biological Activities of Classical
Pathway Components
Component Biological Activity
C2b Prokinin; cleaved by plasmin to yield kinin, which
results in edema
C3a Anaphylotoxin; can activate basophils and mast
cells to degranulate resulting in increased vascular
permeability and contraction of smooth muscle cells,
which may lead to anaphylaxis
C3b Opsonin
Activation of phagocytic cells
C4a Anaphylaotoxin
C4b Opsonin

13. 13
Control of Classical Pathway
Components
Component Regulation
All C1-inhibitor (C1-INH); dissociates C1r and C1s from
C1q
C3a C3a-inactivator (C3a-INA; Carboxypeptidase B)
C3b Factors H and I; Factor H facilitates the degradation
of C3b by Factor I
C4a C3a-INH
C4b C4 binding protein (C4-BP) and Factor I; C4-BP
facilitates degradation of C4b by Factor I; C4-BP
also prevents the association of C2a with C4b thus
blocking formation of C3 convertase

14. C1-inhibitor deficiency:
hereditary angioedema

15. Components of mannose-binding
lectin pathway
MBL MASP1

16. Mannose-binding lectin pathway
MBL
_____
C4b2a is C3 convertase; it
will lead to the generation of
C5 convertase
MASP1

17. Components of the
alternative pathway
C3

18. Spontaneous C3 activation
C3 i
Generation of C3 convertase
C3iBb complex has a very short half life
b
C3b

19. b
C3b
If spontaneously-generated
C3b is not degraded
C3-activation
the amplification loop
C3b

20. C3b
C3 b
C3-activation
the amplification loop
C3b
b

21. C3b
C3b
C3 b
b
C3-activation
the amplification loop
C3b

22. C3b
C3b
C3-activation
the amplification loop
C3b
C3b

23. C3b
C3-activation
the amplification loop
C3b
C3b

24. Control of spontaneous
C3 activation via DAF
C3b
DAF prevents
the binding of
factor B to C3b
Autologous cell membrane
CR1

25. Control of spontaneous
C3 activation via DAF
DAF dislodges
C3b-bound
factor Bb
b
b C3b
Autologous cell membrane
CR1

26. Autologous cell membrane
C3b
C3b
iC3b
Control of spontaneous
C3 activation via CR1
CR1
CR1

27. Degradation of spontaneously
produced C3b
C3b
C3b
iC3b
iC3b C3dg
C3dg
C3c C3c

28. C3b stabilization and
C5 activation
C3b
C3b finds an activator
(protector) membrane
C3b
b
This is stable C5 convertase
of the alternative pathway

29. C3b regulation on self and
activator surfaces
C3b

30. C5-convertase of the two
pathways
C3b C3b
C5-convertase of the
Alternative Pathway
C4b
C3b
C5-convertase of the
Classical and lectin
Pathways

31. Generation of C5 convertase
leads to the activation of the
Lytic pathway
Lytic pathway

32. Components of the lytic pathway
C6
C
9
C7

33. Lytic pathway
C5-activation
C3b
C4b
b

34. Lytic pathway
assembly of the lytic complex
b
C6
C7

35. Lytic pathway:
insertion of lytic complex into cell membrane
b
C6
C7
C
9
C
9
C
9
C
9C
9
C
9 C
9
C
9
C
9

36. Biological effects of C5a

37. Product Biological Effects Regulation
Biological properties of C-activation
products
C2b
(prokinin)
edema C1-INH
C3a
(anaphylatoxin)
mast cell degranulation;
enhanced vascular
permeability;
anaphylaxis
carboxy-
peptidase- B
(C3-INA)

38. Product Biological Effects Regulation
Biological properties of C-activation
products
as C3, but less
potent
(C3-INA)
C4a
(anaphylatoxin)
opsonization;
phagocytosis
C4b
(opsonin)
C4-BP,
factor I
C3b
(opsonin)
opsonization;
phagocyte activation
factors H & I

39. Product Biological Effects Regulation
Biological properties of C-activation
products
anaphylactic as C3, but
much more potent;
attracts & activates PMN
causes neutrophil
aggregation, stimulation
of oxidative metabolism
and leukotriene release
C5a
(chemotactic
factor)
carboxy-
peptidase-B
(C3-INA)
C5b67 protein-S
chemotaxis, attaches
to other membranes

40. 40
Complement Deficiencies and Disease
Classical Pathway
Pathway Component Disease Mechanism
C1INH Hereditary
Angioedema
Overproduction of C2b
(prokinin)
C1, C2, C4 Predisposition
to SLE
Opsonization of immune
complexes help keep
them soluble, deficiency
results in increased
precipitation in tissues
and inflammation

41. 41
Complement Deficiencies and Disease
Lectin Pathway
Pathway Component Disease Mechanism
MBL Susceptibility to
bacterial infections
in infants or
immunosuppressed
Inability to initiate
lectin pathway

42. 42
Complement Deficiencies and Disease
Alternative Pathway
Pathway/Component Disease Mechanism
Factors B or D Susceptibility
to pyogenic
(pus-forming)
bacterial
infections
Lack of sufficient
opsonization of bacteria
C3 Susceptibility
to bacterial
infections
Lack of opsonization and
inability to utilize the
membrane attack pathway
C5, C6, C7 C8, or
C9
Susceptibility
to Gram-
negative
infections
Inability to attack the outer
membrane of Gram-
negative bacteria

43. 43
Complement Deficiencies and Disease
Alternative Pathway cont.
Pathway Component Disease Mechanism
Properdin (X-linked) Susceptibility
meningococcal
meningitis
Lack of opsonization of
bacteria
Factors H or I C3 deficiency
and
susceptibility to
bacterial
infections
Uncontrolled activation of
C3 via alternative
pathway resulting in
depletion of C3