Changes

** The surfaces of certain activators therefore stabilise the usually short-lived C3b in the absence of antibody.

** The surfaces of certain activators therefore stabilise the usually short-lived C3b in the absence of antibody.

*** Gram-negative bacteria, yeasts and fungi are the most efficient activators.  

*** Gram-negative bacteria, yeasts and fungi are the most efficient activators.  

 

* When active C3b is bound to particle surfaces, it is protected from inactivation by another complement component, properdin.

* When bound to particle surfaces on particle surfaces the active C3b is protected from inactivation by another complement component – properdin and can then bind another complement component - Factor B. This produces the complex C3bB. This complex is the only substrate for plasma enzyme - Factor D. This splits a small peptide from Factor B (the Ba peptide) and thus generates an active C3 splitting enzyme - C (the bar indicates an active enzyme).  C  is an enzyme whose substrate is C3, and therefore it generates more C3b  - that generates more C - an extremely efficient positive feedback loop. C can also bind C3b to form the complex enzyme; C . This is one of the two enzymes that activates the Membrane Attack Complex (MAC) by splitting C5 into C5a (a small peptide) and C5b (the initiator of the MAC). At the same time the complement inhibitors Factors I and H are acting to breakdown C3b to iC3b (whether as single C3b or in the complex (C ) in plasma or on bacterial surfaces; although iC3b is inactive in participating in the complement cascade it is the major target for phagocytes – as these cells have large numbers of cellular receptors for iC3b (complement receptors) that they use to promote phagocytosis (opsonization).  

** C3b can then bind Factor B.  

*** This produces the complex C3bB.  

* C3bB is the only substrate for a plasma enzyme known as Factor D.  

** Factor D splits a small peptide (the Ba peptide) from Factor B.

*** This splitting generates an active enzyme that splits C3.

- C (the bar indicates an active enzyme).  C  is an enzyme whose substrate is C3, and therefore it generates more C3b  - that generates more C - an extremely efficient positive feedback loop. C can also bind C3b to form the complex enzyme; C . This is one of the two enzymes that activates the Membrane Attack Complex (MAC) by splitting C5 into C5a (a small peptide) and C5b (the initiator of the MAC). At the same time the complement inhibitors Factors I and H are acting to breakdown C3b to iC3b (whether as single C3b or in the complex (C ) in plasma or on bacterial surfaces; although iC3b is inactive in participating in the complement cascade it is the major target for phagocytes – as these cells have large numbers of cellular receptors for iC3b (complement receptors) that they use to promote phagocytosis (opsonization).  

The main effects of alternative complement activation are; (1) to coat bacteria with iC3b which is a major target for phagocytosis by macrophages and neutrophils via the complement receptors, and (2) to induce an acute inflammatory response via C3a and C5a. These ‘anaphylatoxins’ are chemotactic for neutrophils and induce the production of the cytokines that are responsible for acute inflammatory (IL-1β and TNFα).

The main effects of alternative complement activation are; (1) to coat bacteria with iC3b which is a major target for phagocytosis by macrophages and neutrophils via the complement receptors, and (2) to induce an acute inflammatory response via C3a and C5a. These ‘anaphylatoxins’ are chemotactic for neutrophils and induce the production of the cytokines that are responsible for acute inflammatory (IL-1β and TNFα).