Changes

* Some substances can interact to alter aspects of the complement system.

* Some substances can interact to alter aspects of the complement system.

** '''Bacterial cell surfaces'''.

** '''Bacterial cell surfaces'''.

However, bacterial cell surfaces protect the reactive C3b from this rapid inactivation (it is still inactivated but more slowly) and this is the basis of the archaic complement system that is now termed the alternative pathway of complement activation. Subsequently the immune system has optimized complement activation via antibody - antigen complexes; this is now termed the classical pathway of complement activation. The difference between the two pathways only involves the mechanisms of C3 breakdown to C3b. Once C3 is hydrolysed, the later terminal pathway is the same and is often called the membrane attack complex or MAC and involves the binding of C5-C9. The complex sequence of events in both the alternative and classical pathway of complement activation (or complement fixation) is shown in Figure 1.

*** Protect the reactive C3b from rapid inactivation.

**** It is still inactivated, but more slowly.

*** This is the basis of '''alternative pathway''' of complement activation.

**** This is the archaic complement system.

*** The immune system has optimized complement activation via antibody-antigen complexes.

*** This is termed the '''classical pathway''' of complement activation.  

* The difference between the alternative and classical pathways is in the mechanisms of C3 breakdown to C3b.  

** Once C3 is hydrolysed, the terminal pathway is the same for both mechanisms.

*** Often called the membrane attack complex (MAC).

*** Involves the binding of C5-C9.  

===ALTERNATIVE PATHWAY ACTIVATION===

===ALTERNATIVE PATHWAY ACTIVATION===