The complement system as a part of innate immunity is considered to provide rapid tough incomplete antimicrobial activity. However, besides providing a first-line defence, innate immunity plays additional important roles: it initiates and improves the slower, but more specific, acquired immune response. The recognition and destruction of noxious substances, as well as initiation of the acquired immune response, are accompanied by potentially hazardous inflammation. The inflammatory process has thus to be tightly regulated. In this overview, innate immunity and its interactions with acquired immunity are discussed with the main focus on the complement system. Our scientific interests are integrated into the discussion on complement.