The complement system simply explained (Attention: very simplified!):

The complement belongs to the innate immune defense and is thus part of the immune system in humans. About 25 plasma proteins (proteins in the blood) are directly or indirectly involved in the complement system. With activation of the complement system (see below), the complement cascade occurs, which means that the subsequent responses are contiguous, constructive, fast, and irreversible.
The complement system fulfills the following function for the immune system:
mark of pathogens (opsonization) so that macrophages recognize the pathogens as foreign cells.
resolution (Lysis) of the circulating in the circulating antigen-antibody complexes.
inflammation (Inflammatio) of the body tissue to be able to remove the foreign body faster from the bloodstream.
Conclusion: The goal of the activation of the complement system is the detection, the dissolution and the removal of a foreign body cell.


There are three different activation pathways that activate the complement system:
1. The classic activation route
Trigger: Antigen-antibody complex
2. The Lectin pathway
Trigger: Mannose-binding lectin (MBL)
3. The alternative activation route
Trigger: pathogenic surfaces