The complement system is a group of proteins that are important in defense against infection and play a role in the development of inflammation. There are nine major complement proteins called C1 through C9. Complement component C3 is the most important and abundant protein in the complement system. This blood test measures component 3 (C3) of the complement system. Deficiency of C3 may be associated with active Lupus, recurrent bacterial infections, and acute glomerulonephritis (a type of kidney disease caused by problems with body’s immune system). Fasting is not required. Take all medications as prescribed.