This product recognizes the secreted protein complement component 4d (C4d). The complement 1 complex cleaves complement 4 (C4) to form C4b and C4a. C4b levels are strictly regulated. Single site cleavage of the C4b❏'s alpha chain by Factor I forms iC4b and blocks C3 convertase, inhibiting opsonisation and activation of the classical pathway. This requires C4 binding protein or CR1 as a cofactor. iC4b is further degraded into C4d and C4c. C4d❏'s short half life means that C4d is present in serum at high enough concentrations to make it a useful marker for classical complement activation.

C4 has been linked to susceptibility to systemic lupus erythematosous.

C4d