As one of the most ancient parts of the immune system, present in evolution long before the development of adaptive immunity. It efficiently protects the host from pathogenic microorganisms, contributes to immune complex regulation and represents an important link between the innate and the specific immune system.Complement activation is initiated via three distinct routes: the Classical, the Alternative, and the Lectin pathways which unite to trigger covalent deposition of Complement proteins onto the target surface with the ultimate goal to destroy the target either by lysis or phagocytosis. The Classical pathway requires immunoglobulins IgM and IgG for its activation. The Alternative pathway provides a rapid, antibody-independent route for complement activation. Factor B, a single chain plasma protein, binds to C3b which renders Factor B susceptible to cleavage by Factor D, a serine protease circulating in serum in active form.  
The Lectin pathway provides another antibody-independent way to activate the Complement System. This pathway is initiated either by binding of Mannan Binding Lectin (MBL) to mannose or N-acetylglucoseamine or by binding of Ficolins to acetylated carbohydrates on the surface of pathogens. In order to avoid a deleterious damage to surrounding tissue, both soluble and membrane-bound regulators tightly regulate the complement system. C1-inhibitor binds to C1r, C1s and MASP-2 and prevents further cleavage of C4. C4b-binding protein (C4bp) inactivates the classical pathway C3 convertase and the Factors H and I inhibitthe alternative pathway C3 convertase by cleaving C3b in the enzyme complex.

References

1.Mamidi S, et al. Immunobiology. 2017 Jan;222(1):45-54.
2.Gadjeva M,et al. Methods Mol Biol. 2014;1100:1-9.