The part of the immune system, which improves the abilities of antibodies and phagocytic cells that eliminate microbes and destroyed cells from an organism, is known as the complement system. It also promotes inflammation and damages the pathogen's cell membrane. It is a continuous process and hence belongs to the innate immune system. It is non-adaptive and has a lifelong presence. The complement system can be used by antibodies, which are produced by the adaptive immune system. Various small proteins are included in the complement system which synthesize through the liver and circulate as inactive precursors in the blood. There are three biochemical pathways which contribute in the activation of complement system, namely, the alternative complement pathway, the classical complement pathway and the lectin pathway. Such selected concepts that redefine the study of the human complement system have been presented in this book. It aims to serve as a resource guide for students and experts alike and contribute to the growth of research in this area of study. Those in search of information to further their knowledge will be greatly assisted by this book.