1. Complement

2. OBJECTIVES: When you finish this section, you should be able to:
THE COMPLEMENT SYSTEM
OBJECTIVES: When you finish this section, you should be able to:
1. Describe the effects of complement activation.
2. Outline the Classical, Mannan-Binding (MB) Lectin and Alternative pathways of complement activation, including their activators.
3. Discuss the major regulatory points in complement activation and the consequences of deficiencies in complement or complement regulators.
4. Discuss the functions of the various complement receptors.
5. Discribe the biologic activities of complement and identify the components involved.

3. 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
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.

4. COMPLEMENT
ƒ Complex series of plasma proteins (C1-C9).
ƒ Heat labile (inactivated at 56 C for 30 min).
ƒ Synthesized in vivo by liver macrophages,
hepatocytes and intestinal epithelia.
ƒ Can be synthesized in vitro by monocytes and
macrophages.
ƒ C3 is most abundant (1g/L of plasma) and important complement component.
ƒ Complement system ;
Plays a major role in innate and adaptive
immunity.

6. Complement functions Host benefit: Host detriment:
opsonization to enhance phagocytosis
phagocyte attraction and activation
lysis of bacteria and infected cells
clearance of immune complexes
clearance of apoptotic cells
Host detriment:
Inflammation, anaphylaxis

8. 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)

9. Definitions
C-activation: alteration of C proteins such that they interact with the next component
C-fixation: utilization of C by Ag-Ab complexes
Convertase/esterase: altered C-protein which acts as a proteolytic enzyme for another C-component

10. 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)

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

12. THE CLASSICAL COMPLEMENT PATHWAY
ƒ Activated by Ag-Ab binding (can also be activated by viruses, Mycoplasma, DNA).

14. ƒ One IgM molecule can stimulate C1 since it is a pentamer with 5 Fc regions. At least 2 IgG molecules
are required to activate C1.
ƒ IgM more efficient complement binding (fixing) antibody than IgG.
ƒ IgA and IgE lack C1q receptors and cannot activate complement.
ƒ Activated C1q activates C1r which activates C1s.

19. Components of the Classical Pathway
C1r
C1s
C1q C4 C2 C3
Chelating agents dismantle the C1 complex and are anti-complementary. Heat destroys the C2 component. Sample for C measurement should be drawn in a green-top vial (no EDTA), must be kept cold and tested as soon as possible.
C1 complex

20. Classical Pathway Generation of C3-convertaseb
C1r
C1s
C1q C4

21. Classical Pathway Generation of C3-convertase
C2b a
C4a
C1r
C1s
C1q
_____
C4b2a is C3 convertase
C4b

22. Classical Pathway Generation of C5-convertase
C2b
C4a
C1r
C1s
C1q
C3a b
________
C4b2a3b is C5 convertase; it leads into the Membrane Attack Pathway
C4b C3 C2 a

23. Biological Activities of Classical Pathway Components
Biological Activity
C2a(or b)
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

24. Control of Classical Pathway Components
Regulation
All
C1-inhibitor (C1-INH); dissociates C1r and C1s from C1q
C3a
C3a-inactivator
C3b
Factors H and I; Factor H facilitates the degradation of C3b by Factor I
C4a
C3a-INH

25. C1-inhibitor deficiency: hereditary angioedema

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

27. Components of the lytic pathway9

28. Lytic pathway C5-activationb
C5a C5
C3b
C4b C2 a

29. Lytic pathway: insertion of lytic complex into cell membrane9 C 9 C 9 C 9 C 9 C 9 C 9 C 9 C 9

30. MANNAN-BINDING LECTIN COMPLEMENT PATHWAY
ƒ Activated by binding of a serum protein -mannose-binding
lectin (MBL) to mannose-containing proteins or
to carbohydrates on bacteria or viruses.
ƒ Activates the classical pathway in an (antibody and C1)
independent manner.
ƒ MBL is structurally similar to C1q. Instead of C1r and C1s,
MBL associates with mannose-binding lectin-associated
serum proteases (MASP-1 and MASP-2) to activate C4 and C2
ƒ Activation by MBL:MASP complex generates C3 convertase
which progresses as in the classical pathway to produce the
membrane attack complex.

31. Components of mannose-binding lectin pathway
MASP2 C2
MASP1
MBL

32. Mannose-binding lectin pathway
_____
C4b2a is C3 convertase; it will lead to the generation of C5 convertase
C4b
C4a
C2b
C2a
C4b C4
C2a C2
MASP2
MASP1
MBL