Figure 2: Activation and consumption of complement in vivo and in vitro. In vivo, complement is activated, and C3 gives rise to the activation products C3a, C3b/iC3b, C3d,g, and C3c (indicated by arrows). In vivo, a fraction of these complement products are bound to and eliminated by different complement receptor-bearing cells in contact with plasma (a). When blood is drawn in the presence of EDTA, all further complement activation is inhibited (b). The complement system is active in serum and may be activated to a substantial degree in vitro in maltreated samples (c), but it can be kept essentially intact in properly handled samples (d). The thickness of the arrows in each panel indicates the degree of C3 cleavage.