The human organism is constantly exposed to microbes and infectious agents and consequently has developed a complex and highly efficient immune defense which is aimed to recognize and eliminate such infectious agents. The response of the human host to infectious agents forms a double edged sword of immunity. The immune system has to keep a tight balance between attack on foreign surfaces and protection of host surfaces. In its proper function the immune response is aimed to recognize, attack and eliminate invading infectious agents and this response is beneficial for the host. However when the activated immune response like the complement system is not properly controlled and deregulated, effector compounds can attack and damage self-surfaces and this results in disease. In addition pathogens which cause infections and disease protect themselves from the damaging and harmful host immune weapon and use specific immune escape strategies. The complement system forms the first defense line of innate immunity and aids in the elimination of microbes and modified self-cells. Defective regulation of this cascade type system results in infections and in pathology. This can result in diseases, like severe renal diseases hemolytic uremic syndrome (HUS) and dense deposit disease (DDD), in age related macular degeneration a common form of blindness and also in other forms of autoimmune diseases.