It covers the basic concept on Complement System with some test questions.

1. THE COMPLEMENT
SYSTEM
BY MUSOMA JESSE

2. INTRODUCTION
The complement refers to a series of factors
occurring in normal serum that are activated
characteristically by antigen-antibody interaction,
and subsequently mediate a number of biologically
significant consequences

3. Synthesis and Denaturing
Complement proteins are synthesized
mainly by the liver.
Complement C1 is synthesized in
intestinal epithelium.
C2,4 is synthesized in macrophages.
C5,8 is synthesized in spleen.
C3,6,9 is synthesized in liver.

4. Denaturing
Complement is heat-labile; i.e., it is
inactivated by heating serum at 56°C for 30
minutes

5. Biological Effects
Cytolysis of cells such as bacteria,
allografts, and tumor cells; e.g C5, 6, 7, 8,
9.
Opsonization, i.e., enhancement of
phagocytosis
Enhancement of Antibody Production
Anaphylaxis e.g C3a and C5a
Chemotaxis e.g C5a

6. The complement system

7. Terms used in complement
C- activation
C – fixation
Haemolytic unit
C – deactivation
Convertase or Esterase

8. Complement Proteins
It consists of approximately 20 proteins that
are present in normal human (and other
animal) serum
C1, C2 , C4,C5,C6,C7,C8,C9
Factors ( B, D, H,I and P)
MBL, MASP-1 and MASP-2
C1 INH , C4BP,DAF
CR1 and Protein S (vitronectin)

9. Nomenclature of complement
• Over lining
• Large fragments
• Smaller fragments
• Letters ‘a’ and ‘b’
• EXCEPTION C2

10. Nomenclature and activation
• Complement proteins are often
designated by an uppercase letter C and
are inactive until they are split into
products.
– Example: C1
• When the products are split they become
active. The active products are usually
designated with a lower case a or b.
– Example: C1a and C1b

11. The complement system
• A defensive system consisting of over 30
proteins produced by the liver and found in
circulating blood serum.
• Complement kills microbes in three
different ways
– 1. opsonization
– 2. inflammation
– 3. Cytolysis

12. A Cascade system
• The complement works as a cascade
system.
– Cascade is when one reaction triggers
another reaction which trigger others and so
on. These types of systems can grow
exponentially very fast.

13. Complement Pathways
• The complement pathway can be activated by
either of three different pathways.
– Classical pathway (specific immune system)
– Lectin
– alternative (non-specific immune system)

14. The Classical Pathway
• The classical pathway is
considered to be part of
the specific immune
response because it
relies on antibodies to
initiate it.
• Only IgM and IgG fix
complement
• C1 becomes activated
when it binds to the ends
of antibodies

15. The building of a C3 activation
complex
• Once C1 is activated, it
activates 2 other complement
proteins, C2 and C4 by cutting
them in half
• C2 is cleaved into C2a and
C2b
• C4 is cleaved into C4a and
C4b
• Both C2b and C4b bind
together on the surface of the
bacteria
• C2a and C4a diffuse away

16. C3 Activation complex
• C2b and C4b bind
together on the surface
to form a C3 activation
complex
• The function of the C3
activation complex is to
activate C3 proteins.
– This is done by cleaving
C3 into C3a and C3b

17. C3b
• Many C3b molecules are produced by the C3
activation complex.
• The C3b bind to and coat the surface of the
bacteria.
• C3b is an opsonin
– Opsonins are molecules that bind both to
bacteria and phagocytes
– Opsonization increases phagocytosis by
1,000 fold.
Bacteria
Opsonins

18. C3a
C3a increases the inflammatory response by
binding to mast cells and causing them to
release histamine

19. Building the C5 activation complex
• Eventially enough C3b is cleaved that the
surface of the bacteria begins to become
saturated with it.
• C2b and C4b which make up the C3
activation complex has a slight affinity for
C3b and C3b binds to them
• When C3b binds to C2b and C4b it forms
a new complex referred to as the C5
activation complex

20. Overview

21. The alternative pathway
• The alternative pathway is part of the non-
specific defense because it does not need
antibodies to initiate the pathway.
• The alternative pathway is slower than the
Classical pathway

22. Points to note:
Amplification loop
Factors involved
Generation of C3 convertase and C5
convertase
Control of the loop and the factors
involved
Stabilization of the loop

23. The Alternative
complement pathway

24. Initiation of The Alternative pathway
• C3 contains in unstable
thioester bond.
• This unstable bond
makesC3 subject to slow
spontaneous hydrolysis to
C3b and C3a
• The C3b is able to bind to
foreign surface antigens.
• Mammalian cells contain
sialic acid which inactivates
C3b

25. Factor B
• C3b on the surface of
a foreign cells binds
to another plasma
protein called factor B

26. Factor D
• The binding of C3b to
factor B allows a
protein enzyme called
Factor D to cleave
Factor B to Ba and
Bb.
• Factor Bb remains
bound to C3b while
Ba and Factor D
disperse away.

27. The C3 activation complex
• Properdin, also called factor P, binds to the
C3bBb complex to stabilize it.
• C3bBbP make up the C3 activation complex for
the alternative pathway

28. The C3 activation Complex
• The C3 activation
complex causes the
production of more
C3b.
• This allows the initial
steps of this pathway
to be repeated and
amplified
• 2X106
molecules can
be generated in 5
minutes

29. C5 activation complex
• When an additional C3b
binds to the C3 activation
complex it converts it into
a C5 activation complex.
• The C5 activation
complex cleaves C5 into
C5a and C5b.
• C5b begins the
production of the MAC.

30. Overview

31. • The lectin pathway is very similar to the classical
pathway.
• It is initiated by the binding of mannose-binding lectin
(MBL) to bacterial surfaces with mannose-containing
polysaccharides (mannans).
• Binding of MBL to a pathogen results in the association
of two serine proteases, MASP-1 and MASP-2 (MBL-
associated serine proteases).
• MASP-1 and MASP-2 are similar to C1r and C1s,
respectively and MBL is similar to C1q.
Lectin pathway

32. • Formation of the MBL/MASP-1/MASP-2 tri-molecular
complex results in the activation of the MASPs and
subsequent cleavage of C4 into C4a and C4b. The C4b
fragment binds to the membrane and the C4a fragment is
released into the microenvironment.
• Activated MASPs also cleave C2 into C2a and C2b. C2a
binds to the membrane in association with C4b and C2b is
released into the microenvironment.
• The resulting C4bC2a complex is a C3 convertase, which
cleaves C3 into C3a and C3b.
Lectin pathway

33. • C3b binds to the membrane in association with C4b and
C2a and C3a is released into the microenvironment. The
resulting C4bC2aC3b is a C5 convertase. The generation of
C5 convertase is the end of the lectin pathway.
• The biological activities and the regulatory proteins of the
lectin pathway are the same as those of the classical
pathway.
Lectin pathway

34. Over view of Lectin pathway

35. The C5 activation complex
• The C5 activation complex (C2b, C4b,
C3b) activates C5 proteins by cleaving
them into C5a and C5b
• Many C5b proteins are produced by the
C5activation complex. These C5b begin
to coat the surface of the bacteria.

36. The function of C5a
• C5a disperses away from the bacteria.
– Binds to mast cells and increases inflammation.
– Most powerful chemotactic factor known for leukocytes
– Stimulates respiratory burst
– It is a potent anaphylatoxin

37. Building the Membrane Attack
complex
• C5b on the surface of bacteria binds to C6
• The binding of C6 to C5b activates C6 so
that it can bind to C7
• C7 binds to C8 which in turn binds to
many C9’s
• Together these proteins form a circular
complex called the Membrane attack
complex (MAC)

38. Membrane Attack complex
• The MAC causes Cytolysis.
– The circular membrane attack
complex acts as a channel in
which cytoplasm can rush out
of and water rushes in.
• The cells inner integrity is
compromised and it dies
• Control of lysis of normal
cells by Protein S
(vitronectin)

39. Compliment pathways

40. Comparisons among the
pathways
i. Antibody initiation
ii. Divalent cation
iii. Prokinin generation
iv. Anaphylatoxin generation
v. C3 convertase and C5 convertase
vi. Feeding into the MAC

41. Problems:
Which of the following is not true about the complement
system?
(a)It is a set of more than 20 proteins that play a key role in host defense by
specifically acting in different ways toward different microorganisms.
(b)Its general functions include enhancing phagocytosis by phagocytes, lysing
microbes and enveloped viruses directly, and generating peptide fragments that
regulate inflammation and immune responses.
(c)It is a fast-acting innate host defense that works in a cascade.
(d)There are two pathways (classical and alternative), with the former beginning
when antibodies bind to microbes which trigger C1, C4, and C2 complement
proteins, and the latter activated by contact between complement protein
factors B, D, P and polysaccharides at the pathogen surface.
(e)The effects of both pathways are the same

42. 2. Describe the complement system,
including the classical and alternative
(properdin) pathways.
3. What are the results of activating the
complement cascade?

43.  C3a and C5a can cause:
(a) bacterial lysis
(b) vascular permeability
(c) phagocytosis of IgE- coated bactreia
(d) aggregation of C4 and C2

44. Complement fixation refers:
a) the ingestion of C3b- coated bacteria by
macrophages
b)The destruction of complement in serum
by heating at 56oC for 30minutes
c)The binding of complement components
by antigen-antibody complexes
d)The interaction of C3b with mast cell

45. THANK YOU