12) Complement Flashcards
3 C’ pathways
- classical
- alternative
- common
current definition of complement
a system of plasma proteins which mediate a variety of inflammatory effects in addition to bacteriolysis
discovered classical pathway
1894
Pfeiffer
discovered alternative pathway
1954
Pillemer
Ab dependent and independent C’ pathways
classical: dependent
alternative: independent
pathway that evolved earlier
more primitive
alternative
why does complement become functional in a cascade?
too destructive to exist in plasma in activated form
3 general functions of C’ system
- lysis of bacteria/enveloped viruses
- opsonization (phagocytes have receptors for C3)
- anaphylatoxins cause inflammation (vasodilation, permeability, chemotaxis)
requirements for classical pathway
- antigenic surface
- Ab bound to antigenic surface (1 IgM, or 2 IgGs closeby)
IgG subclasses best at fixing C’
- IgG3
- IgG1
- IgG2
- IgG4 (not at all)
3 units/phases of classical pathway
- recognition unit (C1 binding, activation)
- activation unit
- MAC
trimolecular complex that initiates classical pathway
components?
C1
C1q
C1r
C1s
Classical Pathway Steps
(Not including common pathway)
- C1q binds Fc of Ab bound to antigenic surface
- C1 is activated, and molecule twists
- Activated C1r then activates C1s
- C1s activates C4, and then (with C4) C2
- Complex C42 forms and attaches to the surface —the C3 convertase
- C3 convertase cleaves C3
- C3a is released; C3b binds to membrane surface (C5 convertase proximal, opsonin distal)
Common pathway begins
Common Pathway Steps
- C5 convertase cleaves C5
- C5a is released; C5b forms focal point for MAC
- C5b fuses with C6 and C7
- Complex attaches to target cell membrane
- C8 and C9 bind, forming transmembrane channel
- Cell lyses
Alternative Pathway Steps
(Not including common pathway)
- Spontaneous cleavage of C3; C3b binds surface
- C3b binds factor B and factor D
- New C3b binds membrane surface proximally; binds properdin to stabilize C5 convertase (distal binding is opsonin)
best C1q binding configuration
IgM + IgG3
C1s activates —–, which is required to then activate —–
C4
C2
determines whether C3b is an opsonin or part of C5 convertase
whether it attaches to membrane proximal or distal to C42/C3bBD
how does MAC cause lysis?
- water flows in
- K+ flows out
- Na+ flows in
- gross swelling of cell
- contents leak out
empty cell sack is phagocytized
requirements for the alternative pathway
- surface with repeating polysaccharide units (bacteria, fungi, viruses)
- spontaneous cleavage of C3 (0.2-0.4% total C3/hour)
functions of anaphylatoxins C3a and C5a
- degranulation of masts (histamine) - smooth muscle contraction, permeability
- induce local edema - fluid contains more Ab and C’
- chemotaxis
examples of C’ system regulators
- C1 esterase inhibitor
- Beta 1 H
- DAF
- cAb inhibitor
- anaphylatoxin inhibitor
- S protein
- C8 binding protein
- C3b inhibitor
indications for C’ lab tests
- suspected hyper-utilization due to complement-activating disease
- suspected hyposynthesis due to genetic deficiency
3 parts of C’ panel
- total hemolytic complement (CH50 assay)
- C3 quantitative
- C4 quantitative
only pathway measured by total hemolytic complement
classical + common
method used for C3 and C4 quantitation
nephelometry
measures presence only, not function
measures rate of Ag-Ab complex formation by measuring amt of light scattered as they are formed
C3/C4 nephelometry
special C’ assays done in reference labs
- all complement components, immunochemical or functional
- C1 esterase inhibitor, immunochemical or functional
measures ability of test sample to lyse 50% of sheep rbcs coated with rabbit anti-srbc
CH50 total hemolytic complement
CH50 steps
- mix srbc-Ab with standard, control or pt serum
- incubate
- centrifuge
- read hemolysis at 415 nm
- calculate CH50 value
CH50 precautions
- collect on ice; C1q and C5 are heat-labile
- reconstitute reagents with cold water
- follow procedure exactly
- pipette carefully
causes of elevated complement levels
- acute inflammatory conditions (acute phase proteins)
- rheumatic diseases
- viral hepatitis
- MI
- cancer
- DM
- pregnancy
- sarcoidosis
- amyloidosis
- thyroiditis
- IBD
- typhoid
- pneumococcal pneumonia
causes of decreased complement levels
- hypercatabolism (severe rheumatic disease, subacute bacterial endocarditis, HBV, GN sepsis, malaria, cryoglobulinemia)
- liver disease (decreased synth)
- protein malnutrition
- genetic deficiency
seriousness of genetic C’ defiency depends on…
whether component is involved in 1 pathway or the common pathway
sx of genetic C’ deficiency
recurrent infections
deficiency of C1 esterase inhibitor
hereditary angioedema (HANE)
uncontrolled activation of C4 and C2, resulting in local edema
life threatening if in larynx
hereditary angioedema (HANE)
HANE tx
- synthetic C1 esterase inhibitor
- non-masculinizing androgenic steroids
