The complement system is the parts of the immune system that assists other immune cells that do destroy foreign cells. Unlike other immune system cells, such as B cells and antibodies, they are not specific to a particular pathogen, nor do they adapt to pathogens or develop during a person's lifetime. They also promote inflammation and assist with the breakdown of the cell wall of foreign cells - the first reaction of the immune system to any foreign pathogen or cell.
Most of the cells of the complement system are created by the liver. Once created, they are inert within the bloodstream until they are triggered by an outside influence. Once triggered, they create cytokines which cause a cascade effect throughout the immune system, activating more and more defences.
It is now believed that several immune system disease are caused by a shortage of complement proteins, such as asthma, multiple sclerosis and inflammatory bowel disease. Other diseases are caused by a lack of the factors that regulate the complement system, such as hemolytic uremic syndrome.