The complement system
What is the complement system? The complement system is a biochemical chain-reaction which starts with the activation of just a few complement factors that in turn activate other nearby factors and so on. Complement factors are made up of more than 30 proteins that are produced by the liver cells and released into the blood. This chain reaction leads to a tremendous amplification of the complement response.
Take a look at this video that illustrates how the complement system works. Can you spot this chain reaction and identify points when the responses can be amplified?
So what is the complement system good for? Well, in the Video you clearly saw how the complement system built up a membrane attack complex (MAC) that bored a hole in the surface of the bacteria causing it to die. But there were another two important ways by which complement helps to eliminate pathogens that were covered in this video. I'll give you a clue. Complement factors are cleaved into large and small fragments. What do the large and the small fragments do? Take another look at the video.
Generally, the large fragments bind pathogens while the small fragments float away and cause inflammation and anaphylatoxic activity. The bound fragments either render pathogens sticky for phagocytic cells that remove the pathogen, or form a MAC that cause pathogens to rupture. The complement system therefore can either hire assassins to kill pathogens via the small fragments or can pull the trigger itself with the larger fragments.
The complement chain-reaction can be started in different ways. In the video you have just seen it is started by binding to antibodies which is called the classical pathway. But it can also be started as the complement fragments break up spontaneously in the alternative pathway or as they bind mannose-binding lectins in the mannose-binding lectin pathway. The end result is the same; small and big fragments helping eliminating pathogens.