Complement C4 is a protein with a molecular mass of 206 kDa. The synthesis of complement C4 takes place mostly in the hepatic parenchyma, but a part is synthesised by monocytes or other tissues.
Complement C4 is the enzyme of activation of complement C3 and C5.
The complement C4 concentration can increase slightly following an acute inflammatory phase.
A decrease in complement C4 concentration can be caused genetically, by hereditary angio-oedema, by autoimmune haemolytic anaemia, or autoimmune nephritis.