The overview Complement has three functions Complement functions

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

The CLASSICAL pathway Stimulated by antibodies: specifically: Ig. M and Ig. G (subclasses 1, 2, 3) Start with C 1 q a HUGE protein (410, 000 daltons!) Composed of 18 peptides. Peptides can associate to form trimers; six sets of trimers make C 1 q 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 C 1 are C 1 r and C 1 s which associate to make dimeric pairs (C 1 r 2 s 2); the dimeric pair joins C 1 q to form C 1 qr 2 s 2.

The CLASSICAL pathway • C 1 qr 2 s 2 binds to TWO immunoglobulins. – The complement binding sites of circulating Ig. M are too far apart to bind complement; – only when Ig. M is bound does it fold so that C 1 qr 2 s 2 can “see” nearby complement binding domains. – Ig. G concentrations must be high in the vicinity of antigens for threshold levels of complement binding domains to be present.

The CLASSICAL pathway When C 1 qr 2 s 2 is bound to requisite number of immunoglobulins, C 1 r “autocatalytically” converts to C 1[r], in turn, converts C 1 s to C 1[s] cleaves C 2 and C 4 is cleaved to C 4 a and C 4 b; C 4 b associates with its “target” which is C 2 is cleaved by C 1[s] making C[4 b 2 a] which is a C 3 convertase! (Note that 2 a is bigger than 2 b, this nomenclature being the on exception to the convention that “a” is smaller than “b. ”) As with the other C 3 convertase, C 3 b can join C[4 b 2 a] to make C[4 b 2 a]3 b which is also a C 5 convertase.

The CLASSICAL pathway

The CLASSICAL pathway

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 C 1 q[r 2 s 2], that is a C 3 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 Bcell 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. . .