C2 and COVID-19: The complement proteins C1q, C2, C4, C4b, MBL, C5, C5a, C3, C3b and complement factors such as factor B, factor D, factor H, and factor I were significantly enhanced in children with severe or moderate COVID-19 (Figure 4; eFigure 3, eTable 5, eTable 6 in Supplement 1).