The overview…. Complement has three functions: –Opsonin –Chemoattractant –Membrane Attack Complex (MAC) Complement functions in two (three?) systems: –Alternative –Classical –Lectin-based

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)

The CLASSICAL pathway Also associated with C1 are C1r and C1s which associate to make dimeric pairs (C1r 2 s 2 ); the dimeric pair joins C1q to form C1qr 2 s 2.

The CLASSICAL pathway C1qr 2 s 2 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 C1qr 2 s 2 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.

The CLASSICAL pathway When C1qr 2 s 2 is bound to requisite number of immunoglobulins, C1r “autocatalytically” converts to C1[r]. C1[r], in turn, converts C1s to C1[s]. C1[s] cleaves C2 and C4. C4 is cleaved to C4a and C4b; C4b associates with its “target” which is C2. C2 is cleaved by C1[s] making C[4b2a] 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 C[4b2a] to make C[4b2a]3b which is also a C5 convertase.

The CLASSICAL pathway

The complement pathways…

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 C1q[r 2 s 2 ], that is a C3 convertase.

The most amazing circumstance: Erythrocytes (!) deliver the complex of antigen – antibody – complement to the liver and spleen for consumption by phagocytes.

Monoclonal Ab’s

Contrast with the normal circumstance… The normal response to an immunogen is polyclonal Ab’s The immunogen most commonly has multiple epitopes The multiple epitopes select multiple B- cell lineages Multiple clones mature to produce Ab’s for the multiple epitopes

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

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...