Test Details: Complement component 4 (C4), is a protein involved in the intricate complement system, originating from the human leukocyte antigen (HLA) system. It serves a number of critical functions in immunity, tolerance, and autoimmunity with the other numerous component. This test measures the amount of C4 protein in blood. Complement C4 levels may be elevated due to certain conditions such as cancer, viral infections, obesity, diabetes, heart disease, etc. Complement C4 levels may be low due to kidney disease or chronic hepatitis.