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Introduction

Complement – A series of serum proteins involved in the effector role of the immune response to pathogens

Made up of approximately 30 circulating and membrane-bound proteins.

Synthesized in the liver and by cells involved in the inflammatory response.

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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

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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

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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 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)

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• C-activation: alteration of C proteins such that they interact with the next component

• C-fixation: utilization of C by Ag-Ab complexes

• 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

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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

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COMPLEMENT

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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 Anaphylotoxin

C4b Opsonin

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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-Schemotaxis, attaches to other membranes

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Control of Classical Pathway Components

Component 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

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

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Biological effects of C5a

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Overview of Complement-mediatedOverview of Complement-mediatedBiological EffectsBiological Effects

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Biological Functions ofBiological Functions ofComplementComplement

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Complement Deficiencies and DiseaseClassical Pathway

Pathway Component Disease Mechanism

C1INH HereditaryAngioedema

Overproduction of C2b (prokinin)

C1, C2, C4 Predisposition to SLE

Opsonization of immunecomplexes help keepthem soluble, deficiencyresults in increasedprecipitation in tissuesand inflammation

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Complement Deficiencies and DiseaseLectin Pathway

Pathway Component Disease Mechanism

MBL Susceptibility to bacterial infections in infants orimmunosuppressed

Inability to initiate lectin pathway

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Complement Deficiencies and DiseaseAlternative 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

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Complement Deficiencies and DiseaseAlternative 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

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Enhancing B Cell Responses to Antigens

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Removal of Immune Complexes

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Removal of Necrotic cells and Subcellular Membranes

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Responses to Viruses

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The Assay Contains Three Components and Three Steps

Components:

1. The test system Incubate serum + test antigen

2. Complement Add complement

3. The indicator system Add the indicator system

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Complement FixationAssay

(positive test)

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Complement Fixation Assay

(negative test)

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The End