In recent years a plethora of data has accumulated directing toward an important role of polypeptides C3a and C5a and its degradation product C5adesArg, summarized as anaphylatoxins (ATs), in microbial host defense and immune regulation. The ATs exert their various biologic functions by interacting with specific C3a- and C5a-receptors present on cells of myeloid origin, epithelial cells, smooth muscle cells as well as on activated B- and T-cells. Activation of AT receptors mediates signal transduction pathways triggering a variety of proinflammatory events. However, by interacting with the cytokine- and chemokine network C3a and C5a exhibit also anti-inflammatory properties. In this review the focus is on the pathogenetic role of the ATs in sepsis, immune complex disease, delayed type hypersensitivity and asthma. Discussed are data from animal models in which the ATs are blocked by specific C3a or C5a inhibitors or from mice with genetic deletions of the specific receptors of either C3a or C5a/C5adesArg.