Component C4 is a 210-kDa disulphide-bonded heterodimeric protein within the complement cascade. It has several critical functions in immunity, tolerance, and autoimmunity with the other immune components. It is a crucial factor in connecting the recognition pathways of the overall system instigated by antibody-antigen (Ab-Ag) complexes to the other effector proteins of the innate immune response. Anti-human C4 can be used to measure complement C4 levels in the patient serum samples.