The complement system is a key component of the body’s innate immune defense. It has an important role as a first-line defense against microorganisms but is also involved in other important functions such as homeostasis, clearance of cellular debris and immune surveillance. Clearly, such a broad range of functions requires a well-balanced interaction with other biological systems in the body. Indeed, the complement system has been likened to a balancing scale that needs to balance between providing sufficient activity to protect the body but not enough to be harmful. A number of genetic and environmental factors can tip the scale from defense to destruction.