Complement C5 (molecular weight app. 190kDa)is a key component of the complement system. This system plays important roles in both innate and adaptive immune response and can produce an inflammatory and protective reaction to challenges from pathogens before an adaptive response can occur. The complement system consist of a complex family of proteins and receptors which are found in the circulation, in tissues and other body-fluids. Excessive and uncontrolled activation/regulation of the complement system have been implicated in various diseases such as asthma, lupus erythematosus, glomerulonephritis, various forms of arthritis, autoimmune heart disease, multiple sclerosis, inflammatory bowel disease, paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome and ischemia-reperfusion injuries, and rejection of transplanted organs. Three major pathways within the complement system have been described, namely the classical, alternative and lectin pathways which are activated by different stimuli. The classical pathway is initiated by Immune complexes; the lectin pathway by surface bound lectins; and the AP by all the surfaces that are not specifically protected against. All the 3 pathways converge on Complement C3. Cleavage of C3 induces, in turn, the cleavage of C5, successively leading to the production of the membrane attack complex (MAC). This protein complex is the final common effector responsible for lysis of susceptible microorganisms or damage cells. Mutations in C5 gene cause complement component 5 deficiency, a disease characterized by recurrent bacterial infections.