C1S and systemic lupus erythematosus: In this manuscript, we revealed that (a) previously annotated human activated MMP‐9 in plasma or serum samples may be C1s; (b) with the use of EDTA/gelatin zymography these entities were identified and discriminated; (c) the EDTA/gelatin zymography technique was optimal in the absence of Ca2+ and (d) we discovered that the levels of C1s were higher in plasma and IC of the SLE patients analyzed versus control individuals.