In this study, we have quantified 14 proteins implicated in the complement cascade, including key components (C1q, C3, C4, C9), regulators (C1 inhibitor, FI, sCR1, factor H [FH], FHR4, FHR125, clusterin), and activation products (iC3b, C5a, TCC) in plasma from individuals with DS (n = 71) and age‐matched healthy controls (n = 46). Here, C9 is linked to Dravet syndrome.