The COMPLEMENT SYSTEM

Complement proteins are synthesized by the liver and circulate within the plasma in inactive forms.
This system also includes regulatory proteins that inhibit their inappropriate activation.
Lastly, some cells possess complement receptors that give a variety of cellular functions to this system.
There are two pathways of complement activation:     1) The Classical Pathway.
                                                                                  2) The Alternate Pathway.

1) CLASSICAL PATHWAY:  

IgM or IgG antibody Molecules, bound to the surface of micro-organisms, activate the complement System.
The complement proteins actually recognize and bind the antibody on the surface of the pathogen.
In this scenario the complement system could be considered as specific, but the antibody brings about the specificity so it merely complements the specific function of antibody. A series of proteins bind to the immune complex (C1, C2, C4), resulting in the formation of C3 convertase activity.

2) ALTERNATE PATHWAY:     

Some proteins of the complement system can recognize and be activated by typical carbohydrate structures on the surface of foreign microorganisms. The combination of proteins (factor D, factor B) form a C3 convertase.

3) Formation of the Membrane Attack Complex (MAC):  

Once the C3 convertases are created (either from the Classical or from the Alternate Pathway) a series of enzymatic cascades
occurs which leads to the formation of the Membrane Attack Complex (MAC). This cascade includes the sequential binding of C'6 (complement protein 6), C'7 and C'8 to the surface of the Foreign cell. This complex will induce the polymerization of multiple C'9 molecules.
These proteins will self-assemble within the plasma membrane of the target cell, and form a pore. These pores allow
the passage of water and salt molecules, which will damage the target cell by osmotic lysis.