Recently, two additional complement activation pathways have been described, i.e. the properdin and the thrombin pathways, both of which will be discussed in more detail below. Figure 1 depicts a rough schematic of the so far known complement activation pathways and of the biological functions of activated complement components. In brief, the three main activation pathways converge in the formation of enzymatic complexes termed the C3 convertases and C5 convertases, which cleave the two main components of the complement system, C3 and C5. The two proteolytic fragments generated by the action of the convertases are the anaphylatoxins C3a and C5a. Both can trigger proinflammatory signaling through binding to their corresponding receptors, the C3a receptor (C3aR) and C5a receptor (C5aR and C5L2), on various myeloid and non-myeloid cells [28, 29, 45, 46] . C5a is a powerful chemoattractant for neutrophils that recruits immune cells to the site of injury and activates cellular attack mechanisms like oxidative burst and lysosomal enzyme release [47, 48] . Furthermore, the anaphylatoxins contribute to the degranulation of mast cells and basophils, induce the expression of adhesion molecules on endothelial cells, cause smooth-muscle contraction and enhance the acute phase response of the liver [48] . The cleavage of C3 by C3 convertases leads to the generation of a second major fragment, C3b, which acts as an opsonin facilitating the removal of bacteria and cell detritus by phagocytic cells [49] . Finally, the formation of C5b by cleavage of C5 initiates the assembly of a multimolecular complex, the MAC (C5b-9), that perforates membranes of bacteria and nucleated cells and causes rapid cell lysis and death [45, 50, 51] .