Introduction• Complement – A series of serum and

membrane expressed 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.

FoundersJules Bordet & Paul Ehrlich

Key role in defense against many foreign invaders.

• Most important functions are:• Production of opsonins• Production of anaphylatoxins• Direct killing of organisms• Enhancing antigen-specific immune

response• Maintaining homeostasis

Complement Activation Pathways

• Involves a “cascade” of successive components.

• Enhances a small initiating signal.

• Components are cleaved into activated fragments.

• Fragments induce intense inflammatory responses to eliminate infectious agents.

Pathways of the complement activation

• Distinct recognition events for each pathway– Classical - C-reactive protein

– Lectin – Mannose-binding lectin (MBL)

– Alternative – serum factors B, D, and P

The Classical Pathway

• Antigen-Antibody complexes are main activators of this pathway.

• Activated by the formation of soluble Ag-Ab complexes or binding of Ab (IgM or IgG) to Ag on a target cell.

• C-reactive protein binds to the surface of many bacteria and are also activators.

Component Protein ComplexC1

The C1 qrs complex

Classical Pathway

C4BP exclusively regulates the classical pathway

IgM Antibody

IgM and IgG moleculesBinding

C1q binds toIgM and IgG molecules

Activation Effectiveness

• IgM is more effective at activating complement than IgG

• C1q binds to the CH2 domain of Ig and requires at least two adjacent Fc regions

• Activation of the Thiol-Ester bond and covalent attachment to antigen

The Lectin Pathway

• Antibody-independent pathway

• Activated by mannose-binding lectin to mannose residues on foreign surface

• Binding activates MASP-1 and MASP-2 that cleave and activate C4 and C2

• Cleaved C4 and C2 generate C3 convertase

• Converges with the classical pathway at activation of C3

Mannose-binding Lectin Pathway

The Alternative Pathway

• Does not require Ag-Ab complex formation

• Initiated by foreign cell surface proteins

• Produces active C3 and C5 convertase

• Active C3 is generated spontaneously

• Host cells regulate the progression

Alternative Pathway

Factor H exclusively regulates the alternative pathway

C3 and C5 convertases of each pathway

Activation of C3• Cleavage of C3 is a critical step in all three

pathways.

• C3 convertases split C3 into two fragments:

C3a---smaller, fluid-phase

anaphylatoxin

C3b---larger, continues the

sequential activation of

successive components

Activities of activated C3

• C3a promotes inflammation

• C3b fixation to surfaces leads to opsonization

• C3b fixation leads to immune complex clearance

• Generation of the C5 Convertase activity

Activation of C5• Cleavage of C5 produces two fragments:

C5a---released into the fluid phase,

potent anaphylatoxin

C5b---binds to the cell surface,

nucleus for binding the terminal

complement components

The Terminal Sequence• Terminal components of the complement

cascade:

C5b, C6, C7, C8, and C9

• Components are common to all pathways

• Bind to each other and form a MAC

• Results in cell lysis

Formation of the membrane attack complex (MAC)

Formation of MAC

Polymerized C9(poly-C9)

Poly-C9

The “MAC” Attack(membrane lesion – side on view)

Regulation Of Complement Activity

• Regulators may:– dissociate the convertase

– cleave the complement component that is left on the cell surface

– Act as a cofactor for this cleavage

Why doesn’t complement attack our own tissues?

• Inhibiting the classical pathway

Biological Activities Of Complement

• Production of Opsonins

• Production of Anaphylatoxins

• Lysis of Cells

• Enhancing B Cell Response to Antigens

• Controlling the Formation and Clearance of Immune Complexes

• Removing Dead and Dying Cells

• Responses to Viruses

Enhancing B Cell Responses to Antigens

Removal of Immune Complexes

Removal of Necrotic cells and Subcellular Membranes

Responses to Viruses

Complement Proteins Neutralize Viruses

Uncoated Epstein-Barr Virus(EBV)

Complement Deficiencies

• SLE – C1, C4, or C2 deficiencies

• Membranoproliferative Glomerulonephritis – C3 deficiency, rare

• Properdin and factor B and D defects

Complement deficiency and disease

• C3 deficiency – severe, recurrent life-threatening infections with encapsulated microbes

• C1 inhibitor deficiency – hereditary angioneurotic edema

• C1, C2, C4 deficiency – autoimmune disease

• DAF deficiency – paroxysmal nocturnal hemogloblinuria (PHN); RBC hemolysis

Detection of complementary deficiency by ELISA

• Complement concentration in plasma reflects activity of the immune system– Activated complement components are unstable and

extensive formation of immune complexes may deplete complement faster than it can be replaced by the liver.

• Complement Fixation Assay– Detects immune complexes

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

Complement FixationAssay

(positive test)

Complement Fixation Assay

(negative test)