In our study, heat-inactivated serum (Figure 1) resembled the serum of patients suffering from systemic lupus erythematosus—an excess of C1q-bearing, circulating immune complexes (CICs) [56,57] and a lack of the classical pathway regulator C1 inhibitor (C1inh) [58,59], which through activation and simultaneous lack of inhibition results in consumption-related hypocomplementemia [60]. Here, SERPING1 is linked to systemic lupus erythematosus.