Complement is an activation system designed for the elimination of pathogens and is activated in three ways, i.e. the classical, alternative and newly discovered lectin pathways. The classical pathway is initiated by the binding of C1 to immune complexes, whereas the alternative pathway is activated by pathogens themselves without involvement of immune complexes. Upon binding of MBL (mannose-binding lectin) to certain carbohydrates on pathogens, the lectin pathway is activated by two C1r/C1s-like serine proteases termed MASP-1 and MASP-2, which are associated with MBL. As a result, C4, C2 and C3 are activated. The lectin pathway plays a crucial role in innate immunity both in vertebrates and in invertebrates. The mechanism underlying lectin pathway activation remains unsolved. From an evolutional point of view, the classical and lectin pathways are closely related.