Complement system

The complement system is a part of the innate immune system and included more than 40 soluble proteins that are ubiquitously present in the body. It has a key role for the metabolic homeostasis, inflammation, and the immune defence against pathogens. 
The complement system is continuously screening the organism for pathogens and damaged or dead cells. It identifies healthy cells by recognizing specific molecules on the cell surface that suppress activity of the complement system. Pathogens, damaged, and dead cells do not express protective molecules thereby being recognized by the complement system.
When the complement system recognizes damaged or dead human cells, it initiates their clearance without activating major immunological pathways.
When the complement system recognize pathogens, damaged or death cells, complement proteins initiate a series of reactions. First they bind to the pathogen, marking it as a signal for elimination by immune cells and thereby induce an inflammatory response. This process is termed opsonisation. In addition, further complement proteins react, forming a complex that can damage the membrane of bacteria (Merle et al., 2015).