The overview

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The overview.

• Complement has three functions
• Opsonin
• Chemoattractant
• Membrane Attack Complex (MAC)

• Complement functions in two (three?) systems
• Alternative
• Classical
• Lectin-based

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The CLASSICAL pathway

• Stimulated by antibodies specifically IgM and
  IgG (subclasses 1, 2, 3)

• Start with C1q a HUGE protein (410,000 daltons!)
• Composed of 18 peptides.
• Peptides can associate to form trimers six sets
  of trimers make C1q.
• C1q has helical stalks and globular heads.
• (N. B. the heads are the carboxy end and the
  stalks are the amino ends)

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The CLASSICAL pathway

• Also associated with C1 are C1r and C1s which
  associate to make dimeric pairs (C1r2s2) the
  dimeric pair joins C1q to form C1qr2s2.

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The CLASSICAL pathway

• C1qr2s2 binds to TWO immunoglobulins.
• The complement binding sites of circulating IgM
  are too far apart to bind complement
• only when IgM is bound does it fold so that
  C1qr2s2 can see nearby complement binding
  domains.
• IgG concentrations must be high in the vicinity
  of antigens for threshold levels of complement
  binding domains to be present.

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The CLASSICAL pathway

• When C1qr2s2 is bound to requisite number of
  immunoglobulins, C1r autocatalytically converts
  to C1r.
• C1r, in turn, converts C1s to C1s.
• C1s cleaves C2 and C4.
• C4 is cleaved to C4a and C4b C4b associates with
  its target which is C2.
• C2 is cleaved by C1s making C4b2a which is a
  C3 convertase! (Note that 2a is bigger than 2b,
  this nomenclature being the one exception to the
  convention that a is smaller than b.)
• As with the other C3 convertase, C3b can join
  C4b2a to make C4b2a3b which is also a C5
  convertase.

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The CLASSICAL pathway
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The CLASSICAL pathway
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The CLASSICAL pathway
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The complement pathways
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The LECTIN pathway

• Lectins are proteins which bind to carbohydrates.
  
• Many bacteria have many mannose residues on their
  surface. The lectin-based complement system
  begins with a mannose-binding protein (MBP).
• MBP reacts, in turn, with a MBP-associated serine
  protease (MASP).
• MASP functions, in effect, like activated
  C1qr2s2, that is a C3 convertase.

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The mostamazingcircumstance

• Erythrocytes (!) deliver the complex of antigen
  antibody complement to the liver and spleen
  for consumption by phagocytes.

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Monoclonal Abs
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Contrast with the normal circumstance

• The normal response to an immunogen is polyclonal
  Abs
• The immunogen most commonly has multiple epitopes
• The multiple epitopes select multiple B-cell
  lineages
• Multiple clones mature to produce Abs for the
  multiple epitopes

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Monoclonals are just what they say they are

• Remember a single B-cell makes a single type of
  antibody
• or, more precisely, a single idiotype
• The B-cell is made immortal
• The clone, if stable, continues to secrete the
  single type of Ab

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The scheme

• HGPRT s hypoxanthine-guanine phosphoribosyl
  transferase
• The enzyme is used in the salvage pathway for
  DNA synthesis
• HAT s hypoxanthine -- aminopterin --
  thymidine medium
• aminopterin inhibits de novo DNA synthesis
• thymidine is needed for pyrimidine synthesis in
  the salvage pathway
• hypoxanthine for purines...