Where can the complement proteins be found?

The complement proteins are found in the blood and tissues. Proteins that make up the body’s complement system are synthesized in the liver from where they move to the blood and tissues. They then circulate throughout the body through the bloodstream in inactive mode until they encounter a foreign invader. On connecting with a pathogen the complement proteins get activated and act collectively to destroy it. 

The complements circulate in the bloodstream normally as inactive precursors and are activated only during inflammatory reactions, when they act collectively to destroy the pathogen.