The document summarizes key aspects of the complement system. It describes the three complement activation pathways - classical, lectin, and alternative. It explains the proteins involved in each pathway and their roles. The classical pathway is activated by antibody binding, while the alternative pathway is antibody-independent and acts as part of innate immunity. Complement activation results in the formation of the membrane attack complex that can lyse target cells.

1. Dr. Meenakshi Sharma
Assistant Professor
Department of Microbiology
Mayo Institute of Medical Sciences

2.  Introduction
 General propertis
 Complement activation
◦ Classical complement pathway
◦ Alternative complement pathway
◦ Lectin complement pathway
 Regulation of complement activation
 Biological effects of complement
 Deficiencies of the complement system

3.  The term complement refers to a system of
factors that occur in normal serum and is
activated characteristically by Ag-Ab
interaction

4.  Buchner (1889, German bacteriologist) was the first
to observe that the bactericidal effect of serum was
destroyed by heating at 55˚C for one hour
Hans Buchner

5.  Pfieffer (1894),German bacteriologist
discovered the phenomenon of
bacteriolysis.
 Observed that live cholera bacteria could
be injected without ill effects into guinea
pigs previously immunised against cholera
 And plasma from these animals added to
live cholera bacteria caused them to
become motionless and to lyse.This could
be inhibited by previously heating
the blood plasma.
 He called this as bacteriolysis and it
became known as the Pfeiffer’s
Phenomenon

6.  Bordet established that
immune bacteriolysis and
hemolysis required two
factors:
◦ A heat stable antibody
◦ A heat labile factor ‘alexine’
 Paul Ehrlich coined the term
complement repalcing the
term alexine because this
factor complemented the
action of antibody.

7.  It is named “complement system” because it was first
identified as a heat-labile component of serum that
“complemented” antibodies in the killing of bacteria.
 Consists of serum and cell surface proteins involved
in defense against pathogens and tissue damage
mediated by antibodies
 The Complement system is the major effector of
humoral branch of immune system.
 Plays major role in both innate and adaptive
immunity.

8.  Non specific
 5% normal serum protein
 Destroyed in 30 min at 56˚ C (Heat labile)
 Bind only to antibodies that have combined
with their antigens
 Only Ig M, Ig G 3,1,2 fix complement
 Binds to Fc portion of Ig
 30 proteins – C components + Properdin +
Control Proteins
 Nine different fractionsC1 to C9

9.  Complement is a complex of nine different
fractions C1-C9
 Fraction C1 occurs as Ca⁺ ion dependent
complex, which on chelation with EDTA yields
3 protein subunits C1q,r & s.
 Fractions are named C1-9 in the sequence of
cascading reaction, except C4 comes before
C2.
 When a fraction acquires enzymatic or
biological activity it is indicated by a bar over
the component number Cī
 Inactivated form is indicated by prefix ‘i’ (iC3b)

10.  They are mainly synthesized by hepatocytes
 Also produced by blood monocytes, tissue
macrophages and epithelial cells of the
gastrointestinal and genitourinary tract.
 Complement proteins: Made as zymogens -
activation by cleavage.
 Example: C4a ---a smaller fragment
C4
C4b--- larger fragment
Exception C2: C2a = large fragment
C2b = small fragment

11.  Normally present in inactive form in body
 Activity is induced by Ag-Ab combination
 Components react in a specific sequence as a
cascade
 C cascade is a series of reactions in which the
preceding components acts as enzymes for the
succeeding components
 Components are cleaved in dissimilar fragments
 Larger fragments join the cascade and smaller
fragments released in the medium possess
biological functions

12.  Lysis of cells and bacteria
 Promotes virus neutralisation
 Opsonisation
 Immune clearance
 Amplification of inflammatory process by
increasing vascular permiability ,release of
histamine from mast cells

14.  C cascade is trigged off by three parallel but
independent pathways
 Differ only in initial steps
 Once C3 activation occurs, the subsequent
steps are common

15.  3 major pathways of complement activation
◦ classical pathway : activated by Ab bound to Ag
◦ alternative pathway: activated on microbial cell
surfaces in the absence of antibody
◦ lectin pathway : activated by plasma lectin that binds
to mannose residues on microbes

16.  Central event in complement activation is
proteolysis of C3 to generate biologically
active products and subsequent covalent
attachment of C3b to microbial cell surfaces
or to Ab bound to Ag

17. Ag + Ab
C1q,r,s Ca++ Ag : Ab complex
Ag: Ab C (C1q,r,s) C1s esterase
C4
C4a
C4b
Ag:Ab C4b

18. Ag: Ab C4b (C2 Convertase)
Mg++
C2
C2a
C2b kinin
C4b2a (C3 Convertase)
C3
C3a C3bAnaphylotoxic
& Chemotactic
C4b2a3b (C5 convertase)
C5
C5a
C5b
C6, C7
C5b67
C8, C9
C5b6-9
Cell damage
or Lysis

19. The classical pathway is initiated by:
1. Ab binding to the pathogen.
2. C1 proteins binds to the Fc of Ab.
C1 Protein
C1q- 18 polypeptides
6 arms with globular heads-Binds Fc
on IgG or IgM to get activated
2 C1r + 2 C1s
are activated by activated C1q.

20. • C1 must bind to two or more Fc portions to
initiate the complement cascade.
The fc portions of soluble pentameric IgM
are not accessible to C1
(a).After IgM binds to surface-Bound
antigens, it undergoes a shape change that
permits C1 binding and activation
(B). Soluble IgG molecules will also not
activate C1 because each IgG has only one
Fc Region
(c) But after binding to cell Surface
antigens, adjacent IgG Fc portions can bind
and activate C1

21. C3b
C3b is an opsonin
C4b-2a-3b functions as the classical C5 convertase
FORMATION OF MEMBRANE ATTACK COMPLEX
Functions of C3a and C5a
 C3a and C5a increases the inflammatory
response by binding to mast cells and causing
them to release histamine.
Most powerful chemotactic factor known for
leukocytes.
C5b initiate formation of MAC

23.  Cleavage of C5 into C5a and C5b.
 C5 (structurally homologous to C3 and C4, lacks internal
thioester bond )
 C5b initiates formation of MAC (complex of C5b, C6, C7, C8
and multiple C9 molecules ) binds to C6, and C7 , recruits
C8 and complex penetrates more deeply into the membrane.
 C9, a pore-forming molecule with homology to perforin.
The complex of C5b678 forms a nidus for C9 binding and
polymerization
 Penetrates membrane bilayers to form pores
 Disrupt the osmotic barrier, leading to swelling and lysis of
susceptible cells

24.  Also called as Properdin pathway
 Part of innate immunity.
 Antibody Independent.
 The alternative pathway is slower than the
Classical pathway.
 Molecules of C3 undergo cleavage at
continuous low level in normal plasma.