The Complement System in Humoral Immunity

1.         Role in Immunity

a.         The complement system consists of approximately 20 proteins that are present in normal human serum.

b.         The term ‘complement’ refers to the ability of these proteins to complement, i.e., augment, the effects of other components of the immune system.

c.         Effects of complement:

i.          lysis of cells such as bacteria, allografts, and tumor cells.

ii.            generation of mediators that participate in inflammation and attract phagocytes.

iii.            opsonization, i.e., enhancement of phagocytosis.

iv.         aids removal of immune complexes.

d.            Complement proteins are synthesized mainly by the liver.

2.            Activation of Complement

a.         Several complement components are proenzymes, which must be cleaved to form active enzymes.

b.            Activation of the complement system can be initiated either by antigen-antibody complexes or by a variety of nonimmunologic molecules, e.g. endotoxin.

c.            Sequential activation of complement components occurs via one of the three pathways.

d.            Classical pathway: activated by antibody binding to antigen.

e.            Alternative pathway:

i.          more important in initial infection since antibody required to trigger alternative pathway is not present.

ii.          initiated when a spontaneously activated complement component binds to the surface of a pathogen.

iii.            provides amplification for classical pathway as one of the activated components of classical pathway also initiate alternative pathway.

f.          Lectin pathway: initiated by binding of a serum lectin, the mannan-binding lectin, to mannose-containing proteins or to carbohydrates on bacteria or viruses.

g.         The pathways lead to the production of C3b.

h.            Functions of C3b:

i.          it combines with other complement molecules to generate C5 convertase, the enzyme that leads to the production of the membrane attack complex.

ii.          it opsonizes bacteria because phagocytes have receptors for C3b on their surface.

2.         Classic pathway

a.         In the classic pathway, antigen-antibody complexes activate C1 to form a protease which cleaves C2 and C4 to form a C4b2b complex.

b.         Only IgM and IgG fix complement: one molecule of IgM can activate complement; activation by IgG requires 2 cross-linked IgG molecules.

c.         C1 has three proteins – C1q, C1s and C1r: it is C1q which binds to the Fc portion of IgG and IgM and cross-link several immunoglobulin molecules.

d.         C4b2b complex is a C3 convertase, which cleaves C3 molecules into 2 fragments, C3a and C3b.

e.         C3a is an anaphylatoxin; C3b forms a complex with C4b2b, producing C5 convertase which cleaves C5 to form C5a and C5b.

f.          C5a is an anaphylatoxin and a chemotactic factor; C5b binds to C6 and C7 to form a complex that interacts with C8 and C9 to produce the ‘membrane attack’ complex which causes cytolysis.

3.            Alternative pathway

a.         Many unrelated cell surface substances, e.g. bacterial lipopolysaccharides, fungal cell walls, and viral envelopes, can initiate the process by binidng C3 (H₂O) and factor B.

b.         This complex is cleaved by a protease, factor D, to produce C3bBb.

c.         This acts as a C3 convertase to generate more C3b.

d.         The alternative pathway can amplify the classical pathway by depositing more C3b molecules on the pathogen.

4.            Function of the Complement

a.            Opsonization:

i.          cells, antigen-antibody complexes and viruses are phagocytized much better in the presence of C3b.

ii.          this is due to the presence of C3b receptors on the surface of many phagocytes.

b.            Chemotaxis:

i.          C5a and C567 complex attract neutrophils.

ii.          they migrate especially well toward C5a. C5a also enhances the adhesiveness of neutrophils to the endothelium.

c.            Anaphylaxis:

i.          C3a, C4a and C5a cause degranulation of mast cells with release of mediators, e.g. histamine, leading to increased vascular permeability and smooth muscle contraction, especially contraction of thebronchioles leading to bronchospasm.

ii.          C5a is by far the most potent of these anaphylatoxins.

iii.            anaphylaxis caused by these complement components is less common than anaphylaxis caused by type I (IgE-mediated) hypersensitivity.

d.            Cytolysis:

i.            insertion of the C5b6789 complex into the cell membrane leads to killing or lysis of many types of cells including erythrocytes, bacteria and tumor cells.

ii.            cytolysis is not an enzymatic process; rather, it appears that insertion of the complex results in distuption of the membrane and the entry of water and electrolytes into the cell.

e.            Enhancement of Antibody Production:

i.          the binding of C3b to its receptors on the surface of activated B cells greatly enhances antibody production compared with that by b cells that are activated by antigen alone.

ii.          the clinical significance of this is that patients who are deficient in C3b produce significantly less antibody than do those with normal amounts of C3b.

iii.         the low concentration of both antibody and C3b significantly impairs host defenses, resulting in multiple, severe pyogenic infections.

f.            Removal of Immune complexes.

5.            Opsonization

a.         The most important action of complement is to facilitate the uptake and destruction of pathogens by phagocytic cells.

b.         This is due to the presence of C3b receptors on the surface of many phagocytes.

c.         The best characterized of these receptors is the CR1 receptor which is expressed on both macrophages and neutrophils.

d.         C3b in Phagocytosis:

i.            encapsulated bacteria resist uptake by neutrophils and avoid engulfment.

ii.          binding of antibody to bacterial surface leads to binding of C3b.

iii.         uptake of bacteria into neutrophil phagosomes mediated by Fc and C3b receptors.

e.         The small complement fragment C5a can also activate macrophages to ingest bacteria coated with complement alone by binding to a specific C5a receptor.

f.          C5a in Phagocytosis:

i.            bacterium is coated with complement and IgM antibody.

ii.          when only C3b binds to CR1, bacteria are not phagocytosed.

iii.         C5a can activate macrophages to phagocytose via CR1 only.

g.         CR2 makes B cells susceptible to the Epstein-Barr virus which binds specifically to CR2 and is the cause of infectious mononucleosis.

6.            Removal of Immune complexes

a.         Many small soluble antigens form antibody-antigen complexes that contain too few molecules of IgG to be readily bound to Fc receptors.

b.         These include toxins bound by neutralizing antibodies and debris from dead microorganisms.

c.         Such immune complexes are found following most infections and antibody responses, and they are removed from the circulation through the action of complement.

d.         The soluble immune complexes trigger their own removal by directly activating complement, so that the activated components C4b and C3b bind covalently to the complex.

e.         The complement coated complex is then cleared from the circulation by the binding of C4b and C3b to CR1 on the surface of erythrocytes.

f.            Degradation of Immune complexes:

i.          the erythrocytes transport the bound complexes of antigen, antibody, and complement to the liver and spleen.

ii.          here, macrophages remove the complexes from the erythrocyte surface without destroying the erythrocyte, and then degrade the immune complexes.

iii.         even larger aggregates of particulate antigen and antibody can be made soluble by activation of the classical complement pathway, and then removed by binding to complement receptors.

g.            Pathology due to immune complexes:

i.          immune complexes that are not removed tend to deposit in the basement membranes of small blood vessels, most notably those of the renal glomerulus where the blood is filtered to form urine.

ii.          in systemic lupus erythematosus, excessive levels of circulating immune complexes cause huge deposits of antigen, antibody, and complement on podocytes (CR1), damaging the glomerulus; kidney failure is the principal danger.

iii.         immune complexes can also be a cause of pathology in patients with deficiencies in the early components of complement.

iv.         such patients do not clear immune complexes effectively and they suffer tissue damage.

7.         Local responses to Infection

a.         C3a, C4a, and C5a are anaphylatoxins that cause degranulation of mast cells with release of mediators, e.g. histamine, leading to increased vascular permeability and smooth muscle contraction.

b.         These changes recruit antibody, complement and phagocytic cells to the site of an infection.

c.         The increased fluid in the tissues hastens the movement of pathogen-containing antigen-presenting cells to the local lymph nodes contributing to the prompt initiation of the adaptive immune response.

d.         C5a also acts directly on neutrophils and monocytes to increase their adherence to vessel walls, their migration toward sites of antigen deposition, and their ability to ingest particles.

8.            Cytolysis

a.            Insertion of the membrane attack complex into the cell membrane leads to lysis of many types of cells – erythrocytes, bacteria and tumor cells.

b.         The insertion of the complex into the lipid bilayer results in disruption of the membrane and entry of water and electrolytes into the cell.

c.         It leads to the loss of cellular homeostasis, the disruption of the protein gradient across the membrane, the penetration of enzymes such as lysozyme into the cell and eventual destruction of the pathogen.

9.            Regulation of Complement system

a.            Complement binding sites on antibody:

i.          the complement-binding site on the heavy chain of IgM and IgG is unavailable to the C1 component of complement if antigen is not bound to these antibodies.

ii.          this means that complement is not activated by IgM and IgG despite being present in the blood.

iii.         binding of antigen to antibody causes a specific conformational change that enables C1 to bind and initiate the cascade.

b.         C1 inhibitor:

i.          it inactivates the protease activity of C1.

ii.            activation of the classical pathway proceeds past this point by generating sufficient C1 to overwhelm the inhibitor.

c.         Binding of Factor H to C3b:

i.          binding of factor H to C3b induces its cleavage by factor I, a protease.

ii.          this reduces the amount of C5 convertase available.

iii.         the alternative pathway can proceed past this regulatory point if sufficient C3b attaches to cell membranes.

d.         Decay-accelerating factor:

i.            protection of human cells from lysis by the membrane attack complex is mediated by decay-accelerating factor (DAF), a glycoprotein located on the surface of human cells.

ii.          DAF acts by destabilizing C3 convertase and C5 convertase, preventing formation of membrane attack complex.

10.            Clinical Aspects

a.            Deficiency of complement components:

i.            inherited deficiency of C5-C8 greatly enhances susceptibility to Neisseria bacteremia and other infections.

ii.            deficiency of C3 leads to severe, recurrent pyogenic sinus and respiratory tract infections.

b.            Deficiency of C1 esterase inhibitor:

i.          this results in angioedema.

ii.          when the inhibitor is reduced, overproduction of esterase occurs.

iii.         this leads to an increase in anaphylatoxins, which cause capillary permeability and edema.

c.            Paroxysmal nocturnal hemoglobinuria: acquired deficiency of DAF on cell surfaces results in an increase in complement-mediated hemolysis.

d.            Transfusion mismatches:

i.          when type A blood is given by mistake to a person who is type B, antibody to the A antigen in the recipient binds to A antigen on the donor red cells.

ii.            complement is activated and large amounts of anaphylatoxins and membrane attack complexes are generated.

iii.         the anaphylatoxins cause shock, and the membrane attack complexes cause red cell hemolysis.

e.         Immune complex disease:

i.            examples: acute glomerulonephritis and systemic lupus erythematosus.

ii.          binding complement attracts neutrophils, which release enzymes that damage tissue.