What is the Complement system?

The complement system is a part of our immune system. They 'complement' the work of the antibodies. The immune system's job is to keep our bodies safe from foreign invaders like viruses and bacteria, and from homegrown terrorists like cancer cells. The immune system is our country's (body) army.

The complement system is part of the innate immune system and is made mostly in the liver. This means that the body is born with this part of the immune system which is encoded into our DNA. It needs no education in the thymus and doesn't need to be exposed to a bacteria in order for it to work. It is also not part of borrowed immunity. The complement system has great potential to be very damaging to the body, so must be tightly regulated. It's made up of about 30 proteins that circulate within plasma.

Testing the body's complement system is something that is frequently done to help doctors diagnose autoimmune disease. There are many parts to the complement system, and it works in a cascade. It is something like a factory with an assembly line. Each station or part, adds onto or subtracts from the part before it, so if something goes wrong anywhere down the line, the next station can't do its work properly. When autoimmunity occurs, it "uses up" certain parts of the complement system and those deficiencies can be tested and monitored by the complement test panel. Where the deficiency is found gives the doctor a clue what might be happening to the immune system and which autoimmune disease you have.

The job of the complement system is twofold - phagocyte signaling that a cell needs to be removed from the body, and lysing (cell destruction by bursting).

There are 3 basic pathways to the complement system - the classic, the alternative, and lectin pathways. Each are triggered and inhibited by different mechanisms. They each target a specific type of invader.

The complement system is activated by antigen associated antibodies - something that is 'not self' is sensed and the complement system is roused into action. Part of the complement system's job is to create helpful inflammation.

The system is also regulated by special proteins called complement control proteins. Without these regulations to control the immune response of the complement system, inflammation would run amok and our immune system would destroy us.

The complement system is thought to be involved in many disease processes including:

Arthritis (various forms)
Asthma
Atypical hemolytic uremic syndrome
Autoimmune heart disease
Decompression sickness (the bends)
Glomerulonephritis
Hereditary angioedema
Inflammatory Bowel Disease
Lupus (SLE)
Macular degeneration
MS
Transplant (organ) rejection

Certain diseases like HIV manipulate the complement system causing further damage to the body.

To read the rest of the series:

12 things about your immune system your doctor didn't tell you
Your thymus - the educator of your immune system