C3 and chronic kidney disease: We measured C4a as a marker of classical and lectin pathway activation, Ba as a marker of alternative pathway activation, and sC5b-9 as a marker of terminal complement activation, we also measured C3a, C5a, and factor H. Plasma levels of Ba (Fig. 1A) were highest in the CKD samples, consistent with previous reports showing that a decline in kidney function is associated with alternative pathway activation [15,20].