The document summarizes the complement system, including its three activation pathways (classical, alternative, and lectin), membrane attack complex, receptors, regulation, associated disorders, and laboratory assessment. It describes how the classical pathway is activated by antigen-antibody complexes, the alternative pathway does not require antibodies, and the lectin pathway is triggered by microbial polysaccharides binding to lectins or ficolins. Regulation is needed to prevent improper activation. Deficiencies can lead to increased infection risk or autoimmune diseases like SLE. Laboratory tests include measuring complement component levels.
2. Outline
• The classical pathway
• The alternative pathway
• The lectin activation pathway
• The membrane attack complex
• Receptors and biologic functions
• Regulation of complement activation
• Disorders associated with complement activation/deficiency
• Laboratory assessment of complement
• Management of complement deficiency
4. Classical Pathway
• Activated primarily by Ag-Ab complexes
• Soluble
• Fixed on the surface of cells
• Deposited on extracellular matrices
Ability to activate complement
IgM > IgG3 > IgG1 > IgG2
- Need 1 IgM or at least 2 IgGs
- Binding site: IgM – CH3 domain
IgG – CH2 domain
Can’t activate complement
IgG4, IgA, IgE
Abbas, AK. Cellular and Molecular Immunology. 9th ed.
Middleton’s Allergy, 9th ed.
5. Classical Pathway
Apoptotic cell
C-reactive protein
Serum amyloid P
β-amyloid
DNA/chromatin
Cytoskeletal filaments
Some pathogens
Can activate the classical pathway
• Persistence of apoptotic debris leads to
the development of autoantibodies
• C1 deficiency in humans is strongly
associated with the development of
systemic lupus erythematosus
Middleton’s Allergy, 9th ed.
7. Classical Pathway
C4b2a = C3 convertase
Bind C3 Catalyze proteolysis
C2a = large fragment
C2b = small fragment
Abbas, AK. Cellular and Molecular Immunology. 9th ed.
Main function of classical pathway achieved:
C3b has been deposited on the surface of
a pathogen where it can act to enhance
phagocytosis
Middleton’s Allergy, 9th ed.
anaphylatoxin anaphylatoxin
8. Classical Pathway
C4a, C3a, C5a = anaphylatoxins
- Induce mast cell degranulation and neutrophil aggregation
Increased vascular permeability, vasodilation
Most potent, most powerful chemotactic factor for neutrophils
Abbas, AK. Cellular and Molecular Immunology. 9th ed.
Middleton’s Allergy, 9th ed.
anaphylatoxin
C4b2a3b = C5 convertase
11. Alternative Pathway
• Does not require preformed antibody
• Basis of the pathway is “C3 tickover”
spontaneous hydrolysis of C3 at a rate
of 0.2% to 0.4% per hour
Abbas, AK. Cellular and Molecular Immunology. 9th ed.
Middleton’s Allergy, 9th ed.
anaphylatoxin
13. Alternative Pathway
Even when C3b is
generated by the classical
or lectin pathway, it can
form a complex with Bb
C3 convertase
Classical pathway
- C4b2a
Alternative pathway
- C3bBb
Abbas, AK. Cellular and Molecular Immunology. 9th ed.
Properdin
- Released by activated
neutrophils
- Only known positive
regulator of complement
18. Lectin Activation Pathway
Triggered by the binding of microbial polysaccharides to
1) circulating lectins or 2) ficolins
Abbas, AK. Cellular and Molecular Immunology. 9th ed.
N-terminal collagen
like domain
C-terminal
carbohydrate
recognition domain
C-terminal fibrinogen-
like domain
N-terminal collagen
like domain
19. MBL also binds to agalactosyl IgG
with high affinity
Produced at times of inflammation
Activate both classical and lectin
activation pathways
Abbas, AK. Cellular and Molecular Immunology. 9th ed.
Middleton’s Allergy, 9th ed.
Lectin Activation Pathway
23. Membrane Attack Complex
Functions of complement system
• Opsonization
• Clearance of apoptotic debris
• Activation of B cells and of T cells
• Lysis of cells and pathogens
Only function that requires
terminal complements
Middleton’s Allergy, 9th ed.
Abbas, AK. Cellular and Molecular Immunology. 9th ed.
25. Membrane Attack Complex
Nucleated cells are resistant to lysis:
1. Because they are metabolically active, they can repair
membrane damage
2. Eukaryotic cells are coated with complement regulatory
proteins that inhibit completion of the lytic process
Susceptible to lysis
- Gram-negative bacteria
- Most enveloped virus
Resistant to lysis
- Gram-positive bacteria
Middleton’s Allergy, 9th ed.
29. FcÎł receptor of phagocyte
simultaneously engaged by
antibody-coated particles
activate the microbicidal
mechanisms of the phagocytes
Binding of C3b- or C4b-coated
particles to CR1
augment
Abbas, AK. Cellular and Molecular Immunology. 9th ed.
30. Degradation by
reticuloendothelial cells of
liver and spleen
• Immune complex clearance
• Destabilization of the C3 and C5 convertase complexes
• CR1-bearing cells in the bone marrow may become
tolerized on exposure to apoptotic cells bound by
complement → defect could lead to lupus
Middleton’s Allergy, 9th ed.
immune complexes are maintained in a soluble state
through the binding of C3b
CR1
*serum sickness
31. The activation threshold for B cells is
dramatically lowered when both the B
cell receptor and CR2 are simultaneously
engaged.
Patients with C3 deficiency also have
compromised antibody response
Middleton’s Allergy, 9th ed.
Abbas, AK. Cellular and Molecular Immunology. 9th ed.
B cell
coreceptor
complex
32. CD11c = marker of dendritic cells
Middleton’s Allergy, 9th ed.
Abbas, AK. Cellular and Molecular Immunology. 9th ed.
33. Anaphylatoxin receptors
Expressed on mast cells, basophils, eosinophils, neutrophils, platelets,
endothelial cells, and smooth muscle cells.
Effects:
- vasodilation
- aggregation of neutrophils
- stimulation of mucus release from goblet cells
- activation of macrophages
Middleton’s Allergy, 9th ed.
34. C5a receptor
- Also express on hepatocytes, lung epithelium, vascular smooth
muscle, umbilical cord epithelium, astrocytes, microglial cells, T cells
- Superior to C3aR in terms of anaphylatoxic activity
C5a
- Most powerful endogenous chemotactic factor for neutrophil
- Also chemotactic agent for monocyte-macrophage
- Overproduction of C5a in sepsis can lead to inhibition of neutrophil
function → paradoxical immunocompromise seen in sepsis
Middleton’s Allergy, 9th ed.
35. Middleton’s Allergy, 9th ed.
C1q receptors
Their function appears to relate to
phagocytosis and clearance of apoptotic and cell debris.
48. Post–cardiac bypass syndrome
• Bypass circuitry is an activator surface
→ alternative pathway
• Activate kinin system, fibrinolysis, platelet activation
• Release large amounts of C5a
• Neutrophils bind to C5a → form aggregates
- interfere circulation
- release reactive oxygen species
• Results: neutropenia, thrombocytopenia, end-organ
dysfunction, hypotension
Middleton’s Allergy, 9th ed.
49. Serum sickness
• When the clearance pathway is overwhelmed, immune
complexes deposit on endothelial cells wherever pressure
or tortuosity allows (usually skin, glomeruli, and synovium).
• An urticarial or vasculitic rash often is the first sign, followed
by adenopathy, fever, arthritis, and proteinuria.
• Lab: hypocomplementemia secondary to consumption
Patho: leukocytoclastic vasculitis with deposition of C3, IgM,
and IgG
Middleton’s Allergy, 9th ed.
50. Nephritic Factors
• A group of autoantibodies against C3 and C5 convertases
• Associated with lipodystrophy, membranoproliferative
glomerulonephritis
• C3 hypocomplementemia is present
Front Immunol. 2019; 10: 886.
51. •Antibodies to C1q:
- hypocomplementemic urticarial vasculitis
- glomerulonephritis
•Antibodies to factor H
•Antibodies to C1 inhibitor
•Local complement activation
• Placenta of pregnant women with antiphospholipid Abs or
Abs associated with SLE
• Burn injury, pancreatitis, crush injury → activator surface
Middleton’s Allergy, 9th ed.
Presentations same as deficiencies
54. C1 deficiency
• Patients with C1q deficiency present almost uniformly with
early-onset SLE.
• Also increased risk of infection
• More dramatic cutaneous and CNS
manifestations
• Similar autoantibody profile,
anti-dsDNA less common
• Less steroid-responsive
Compared to typical SLE
Middleton’s Allergy, 9th ed.
55. C4 deficiency
• Partial C4 deficiency is extremely common
C4A deficiency: 1-2% of general population
up to 15% of patients with SLE
C4B deficiency: 1-2% of general population
• C4A binds protein, C4B binds carbohydrate
• Complete C4 deficiency is rare
50% of patients with complete C4 deficiency have SLE
Milder form of SLE
Severe, early onset, anti-dsDNA less frequent
Middleton’s Allergy, 9th ed.
56. C2 deficiency
• One of the more common complement deficiencies
• Most are asymptomatic
• Approximately half will develop lupus
• 2/3 have a history of invasive bacterial disease
• Common: S. pneumoniae and H. influenzae
Onset in early adulthood
Cerebritis, nephritis, arthritis less common
Anti-Ro Ab extremely common
Anti-dsDNA infrequent
Middleton’s Allergy, 9th ed.
57. C2 deficiency
• Accelerated atherosclerosis
• Role for complement cascade proteins and complement
regulatory proteins in the regulation of low-density
lipoprotein (LDL) and vascular injury
• Likely that all patients with early complement deficiencies
have a similar phenotype
Middleton’s Allergy, 9th ed.
58. C3 deficiency
• Rarest of the early component deficiency
• Most severe phenotype
• Membranoproliferative glomerulonephritis instead of SLE
• Profound predisposition to infection
• Staphylococcal abscess: inability to opsonize and generate C5a
chemotactic factor
• Recurrent sinopulmonary infection: compromised B cell
costimulation
• Systemic infection: complete failure of C3b opsonization
Middleton’s Allergy, 9th ed.
During infections, a vasculitic rash may
appear and symptoms of serum sickness
may occasionally be seen.
59. Mannose-binding lectin deficiency
• 2-7% in general population
• Mutations lead to accelerated clearance, poor function, and
impaired production
• Modest risk for infection
Middleton’s Allergy, 9th ed.
60. Factor I deficiency
• 1. marked susceptibility to infection due to low C3
• Clinical manifestation like C3 deficiency
• Low but not absent CH50 and AH50
• Low C3
• 2. atypical HUS or MPGN II
• Low or normal C3
• Normal factor I level
• 3. autoinflammatory disease
Middleton’s Allergy, 9th ed.
61. Factor H deficiency
• Main phenotypes:
• Infections
• atypical HUS
• glomerulonephritis
• macular degeneration
Middleton’s Allergy, 9th ed.
MCP deficiency
• Atypical HUS:
later onset compared to Factor H or Factor I deficiency
Low but not absent CH50, AH50
Low C3
Normal complement analysis
Mutation analysis required
62. CD59 Deficiency and PHN
• CD59 expresses on hematopoietic cells and endothelial cells
• CD59 deficiency is associated with chronic hemolytic anemia and
recurrent stroke.
• PNH is caused by mutations of PIG-A → protein product
required for GPI-anchored proteins
• DAF, C8 binding protein and CD59 are GPI-anchored proteins that
protect hematopoietic cells from complement-mediated lysis
• Diagnosis by flow cytometry for CD59 or CD55 (DAF)
Middleton’s Allergy, 9th ed.
64. Indications
• Recurrent sinopulmonary infections
• Recurrent sepsis or recurrent systemic infections, particularly on a
background of autoimmune disease (or a family history of
autoimmune disease)
• Single meningococcal infection,
either meningitis or
meningococcemia in
nonendemic area
Middleton’s Allergy, 9th ed
https://wwwnc.cdc.gov/travel/yellowbook/2020/travel-related-infectious-diseases/meningococcal-disease
If a complement deficiency is
identified, vaccinate to prevent future
infection
65. Indications
• Chronic meningococcemia
• Patients with meningococcal disease with an unusual serotype
(serotype X, Y, Z, W135, or 29E in the United States)
• Patients with meningococcal disease on a background of a positive
family history
• Patients with recurrent meningococcal disease
Frequency of complement deficiency 10-50%
Middleton’s Allergy, 9th ed
66. Indications
• Pediatric-onset severe SLE
• Patients with clinical symptoms suggestive of SLE but with
negative results on antinuclear antibody (ANA) and anti-
dsDNA assays
• Membranoproliferative glomerulonephritis and atypical HUS
• Initial screening: CH50 assay, an AH50 assay, and factor H, factor I,
factor B, and C3 levels
• Flow cytometry for MCP on neutrophils if available
• ADAMTS13 activity level and antifactor H antibody measures
Middleton’s Allergy, 9th ed
67. Indications
• Patients with recurrent angioedema in the absence of
allergic reactions
• Patients with a family history of angioedema
• Patients in whom angioedema is preceded by a reticular rash
• Patients who experience angioedema after trauma
Initial test: C4 levels
Superior strategy: C1 inhibitor antigen and functional levels
Middleton’s Allergy, 9th ed
68. Functional Tests of Complement System
• The CH50 tests the functional capability of serum complement
components of the classical pathway to lyse sheep red blood cells
(SRBC) pre-coated with rabbit anti-sheep red blood cell antibody
(haemolysin).
• The assay result reports the dilution of serum capable of lysing 50%
of the sheep cells.
• Similarly, rabbit red cells are used to measure the intactness of the
alternative pathway (AH50).
Middleton’s Allergy, 9th ed
J Vis Exp. 2010; (37): 1923.
69. • Deficiencies of all of the cascade components (except
C9) lead to a CH50 of zero or near zero
• With low levels of CH50 or AH50, the assays should be
repeated, because mishandling of the serum is an
extremely common problem
Functional Tests of Complement System
Middleton’s Allergy, 9th ed
70. Low but not absent CH50
More common: complement consumption
Less common: regulatory protein defects leading to
consumption of C3
• active immune complex disease
• diminished hepatic production related to liver disease
• immaturity of hepatic production seen in young infants
• Factor D deficiency
• Factor H deficiency
• Factor I deficiency
Middleton’s Allergy, 9th ed
71. Shih, A.R. and Murali, M.R. (2015), Laboratory tests for disorders of complement and complement regulatory proteins. Am. J. Hematol., 90: 1180-1186.
Cannot exclude C9,
properdin, MBL,
MASP-2, or ficolin
deficiencies
→ Genetic testing
- C3
- C4
- C1 inhibitor
73. Early Classical Complement Deficiencies
• Major risks of infection: S. pneumoniae and H. influenzae
→ vaccination
→ lifelong prophylactic antibiotics
• Management of cardiac risk factors due to accelerated
atherosclerosis
• C1q:
- produced to large extent by myeloid cells
→ hematopoietic stem cell transplantation may be considered
Middleton’s Allergy, 9th ed
74. C3 deficiency
• Use of IVIG to compensate for compromised B cell function
• Prophylactic antibiotics
• MPGN in C3 deficiency responds to no specific intervention
• Renal transplantation attempted – unknown recurrence risk
Middleton’s Allergy, 9th ed
Nayagam JS. Am J Transplant. 2020 Aug;20(8):2260-2263.
75. Terminal Complement Deficiencies
• Vaccination every 3 years with the polysaccharide
meningococcal vaccine decreases the frequency of
meningococcal episodes to 20% of that for nonvaccinated
individuals
Middleton’s Allergy, 9th ed
76. HUS from Factor H, Factor I, MCP deficiency
• Plasmapheresis and FFP replacement
• Renal transplantation in end-stage renal disease
• Indicated: MCP deficiency
• Not recommended: Factor H deficiency, Factor I deficiency
• Liver transplantation
Middleton’s Allergy, 9th ed