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ComplementDefinition : series of heat-labile serum proteins
Site : serum and all tissue fluids except urine and CSF
Synthesis : in liver appear in fetal circulation during 1st 13 W
Function : Responsible for certain aspects of
immune response and inflammatory response
Activation : antigen-antibody complex or endotoxin, capsule
series of proteins activated sequentially
Inactivation: inhibitors in plasma (short lived)
Biological effects: either beneficial or harmful to hostP G UPADHYAYA, KIMSMICRO,BANGALORE
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Complement: History
Discovered in 1894 byBordet
It represents lytic activityof fresh serum
Its lytic activity is
destroyed whenheated at 56C for 30min
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Properties
Approximately 30 circulatory & membranebound proteins
complement refers to ability of these
proteins to complement, i.e., augment, theeffects of other components of immunesystem
Important component of innate hostdefenses
Synthesized in the liver and by cells
involved in the inflammatory response.P G UPADHYAYA, KIMSMICRO,BANGALORE
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Effects
Lysis : of cells such as bacteria,allografts, and tumor cells.
Generation of mediators that participatein inflammation and attract neutrophils
Opsonization, ie, enhancement of
phagocytosis
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Complement Functions
Host
detriment:Inflammation
Anaphylaxis
Figure-1. Complement has a central role in inflammationas it causes chemotaxis of phagocytes, opsonization
and lysis of pathogens and clearence of immunecomplexes.P G UPADHYAYA, KIMSMICRO,BANGALORE
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Proteins of the complementsystem (nomenclature)
C1(qrs), C2, C3, C4, C5, C6, C7, C8, C9
factors B, D, H and I, properdin (P)
mannose binding lectin (MBL), MBL associatedserine 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)
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C-activation: alteration of C proteins such that theyinteract with the next component
C-fixation: utilization of C by Ag-Ab complexes
Hemolytic units (CH50): dilution of serum whichlyses 50% of a standardized suspension of Ab-coatedr.b.c.
C-inactivation: denaturation (usually by heat) of anearly C-component resulting in loss of hemolytic activity
Convertase/esterase: altered C-protein which acts
as a proteolytic enzyme for another C-component
Definitions
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Activation
several complements are pro-enzymes, whichmust be cleaved to form active enzymes.
Initiated either by antigen-antibody complexesor by a variety of non-immunologic molecules,
eg, endotoxin
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Activation product of complementproteins (nomenclature)
When enzymatically cleaved, the larger moiety,
binds to the activation complex or membraneand the smaller peptide is released in themicroenvironment
Letter b is usuallyadded to the larger,
membrane-binding, peptide and a to thesmaller peptide (e.g., C3b/C3a, C4b/C4a,C5b/C5a), EXCEPTC2 (the larger, membrane-binding moiety is C2a; the smaller one is C2b)
Activated component are usually over-lined: e.g.C1qrs
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Pathways of complementactivation
CLASSICALPATHWAY
ALTERNATIVEPATHWAY
activationof C5
LYTIC ATTACKPATHWAY
antibody
dependent
LECTINPATHWAY
antibodyindependent
Activation of C3 andgeneration of C5 convertase
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Importance
Alternative pathway is more important the firsttime we are infected by a microorganisms,because the antibody required to trigger theclassic pathway is not present.
Both pathways lead to the production of C3b,the central molecule of the complement
cascade. C3b has 2 important functions: (1) It combines
with other complement components to generateC5
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Importance of C3b
(1) It combines with other complement
components to generate C5 convertase, the
enzyme that leads to production of MAC.
(2) Opsonizes bacteria because phagocytes have
receptors for C3b on surfaces
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Components of the ClassicalPathway
C4C3
C1 complex
C1s is an enzyme and cleaves C4 and C2
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Classical PathwayGeneration of C3-convertase
Cleavage of C4 by C1s
produces C4a and C4b
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Classical PathwayGeneration of C3-convertase
C4b
C4b binds C2 andC4b2 is cleaved by
C1s. C2b is releasedbut C2a remainsbound to C4b on thesurface. C4b2a is C3Convertase
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Classical PathwayGeneration of C5-convertase
C4bC3b
________
C4b2a3b is C5 convertase;it leads into the MembraneAttack Pathway
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Biological Activities of ClassicalPathway Components
Component Biological Activity
C2b Prokinin; cleaved by plasmin to yield kinin, whichresults in edema
C3a Anaphylotoxin; can activate basophils and mastcells to degranulate resulting in increased vascularpermeability and contraction of smooth muscle cells,which may lead to anaphylaxis
C3b OpsoninActivation of phagocytic cells
C4a Anaphylotoxin
C4b OpsoninP G UPADHYAYA, KIMSMICRO,BANGALORE
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Control of Classical PathwayComponents
Component Regulation
All C1-inhibitor (C1-INH); dissociates C1r and C1s fromC1q
C3a C3a-inactivator (C3a-INA; Carboxypeptidase B)
C3b Factors H and I; Factor H facilitates the degradationof C3b by Factor I
C4a C3a-INHC4b C4 binding protein (C4-BP) and Factor I; C4-BP
facilitates degradation of C4b by Factor I; C4-BPalso prevents the association of C2a with C4b thusblocking formation of C3 convertase
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C1-inhibitor deficiency:hereditary angioedema
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Components of mannose-bindinglectin pathway
MBL MASP1
Pathogen
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Mannose-binding lectin pathway
MBL
_____
C4b2a is C3 convertase; itwill lead to the generation of
C5 convertaseMASP1
Binding to lectins cause autocatalytic activation
of MASPs which then cleave C4 & C2P G UPADHYAYA, KIMSMICRO,BANGALORE
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Components of thealternative pathway
C3
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Spontaneous C3 activation
C3
Generation of C3 convertase
fB activate fD which then cut fB releasing
Ba, while Bb becomes an active protease
C3Bb complex has a very short half life
b C3b
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bC3b
If spontaneously-generatedC3b is not degraded
C3-activationthe amplification loop
C3b
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C3b
C3b
C3-activationthe amplification loop
C3bb
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C3b
C3-activationthe amplification loop
C3bC3b
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Control of spontaneousC3 activation via DAF
C3bDAF prevents
the binding of
factor B to C3bAutologous cell membrane
CR1
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Control of spontaneousC3 activation via DAF
DAF dislodges
C3b-bound
factor Bb
bb C3b
Autologous cell membrane
CR1
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Autologous cell membrane
C3bC3b
iC3b
Control of spontaneousC3 activation via CR1
CR1CR1
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Degradation of spontaneouslyproduced C3b
C3bC3b
iC3biC3b C3dgC3dg
C3c C3c
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C3b stabilization andC5 activation
C3b
C3b finds an activator(protector) membrane
C3bb
This is stable C5 convertaseof the alternative pathway
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C3b regulation on self andactivator surfaces
C3b
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C5-convertase of the twopathways
C3b C3b
C5-convertase of theAlternative Pathway
C4bC3b
C5-convertase of theClassical and lectin
Pathways
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Generation of C5 convertaseleads to the activation of the
Lytic pathway
Lytic pathway
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Components of the lytic pathway
C6
C9
C7
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Lytic pathwayC5-activation
C3bC4b
b
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Lytic pathwayassembly of the lytic complex
b
C6
C7
C5b first binds C6 and then C7
from the plasma. Membrane bound
C5b67 recruits C8 and C9 to form
the Membrane Attack Complex (MAC)
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Lytic pathway:insertion of lytic complex into cell membrane
b
C6
C7
C9
C9
C9
C9C9
C9 C
9C9
C9
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Biological effects of C5a
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Product Biological Effects Regulation
Biological properties of C-activationproducts
C2b(prokinin) edema C1-INH
C3a(anaphylatoxin)
mast cell degranulation;enhanced vascular
permeability;anaphylaxis
carboxy-peptidase- B
(C3-INA)
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Product Biological Effects Regulation
Biological properties of C-activationproducts
as C3, but lesspotent
(C3-INA)C4a(anaphylatoxin)
opsonization;phagocytosis
C4b(opsonin)
C4-BP,factor I
C3b
(opsonin)
opsonization;
phagocyte activation
factors H & I
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Product Biological Effects Regulation
Biological properties of C-activationproducts
anaphylactic as C3, butmuch more potent;attracts & activates PMNcauses neutrophilaggregation, stimulationof oxidative metabolism
and leukotriene release
C5a(chemotacticfactor)
carboxy-peptidase-B(C3-INA)
C5b67 protein-Schemotaxis, attachesto other membranes
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Complement Deficiencies and DiseaseClassical Pathway
Pathway Component Disease Mechanism
C1INH HereditaryAngioedema
Overproduction of C2b(prokinin)
C1, C2, C4 Predispositionto SLE
Opsonization of immunecomplexes help keepthem soluble, deficiency
results in increasedprecipitation in tissuesand inflammation
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Complement Deficiencies and DiseaseLectin Pathway
Pathway Component Disease Mechanism
MBL Susceptibility tobacterial infectionsin infants orimmunosuppressed
Inability to initiatelectin pathway
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Complement Deficiencies and DiseaseAlternative Pathway
Pathway/Component Disease Mechanism
Factors B or D Susceptibilityto pyogenic
(pus-forming)bacterialinfections
Lack of sufficientopsonization of bacteria
C3 Susceptibilityto bacterial
infections
Lack of opsonization andinability to utilize the
membrane attack pathwayC5, C6, C7 C8, orC9
Susceptibilityto Gram-negativeinfections
Inability to attack the outermembrane of Gram-negative bacteria
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Complement Deficiencies and DiseaseAlternative Pathway cont.
Pathway Component Disease Mechanism
Properdin (X-linked) Susceptibility
meningococcalmeningitis
Lack of opsonization of
bacteria
Factors H or I C3 deficiency
andsusceptibility tobacterialinfections
Uncontrolled activation of
C3 via alternativepathway resulting indepletion of C3