The complement system is an elegant biochemical cascade consisting of around 50serum soluble and membrane-bound proteins that defend the body from pathogens. It is a central part of the innate immune system, and it is regarded as a first-line defence mechanism of the human body against invading pathogens. The complement system can be activated via three pathways, alternative pathway, classical pathway, and lectin pathway. In all three pathways, C5 convertases are produced to cleave C5 into C5band C5a for host defence. In some environments, such as low pH, pro-oxidative and pro-inflammatory environment; instead, C5 can be activated without any cleavage of the C5 but confronted with a conformational change and expresses a C5b-like structure, still containing the C5a-domain bound to the main molecule. In contrast to normal cells, cancer cells prefer to aspirate in the anaerobic pathway even though there is enough oxygen, so they produce lactic acid, which creates an acidic environment in the tumor microenvironment (TME). The TME is further pro-oxidative and pro-inflammatory, which promotes tumor progression. This thesis aim was to simulate the TME ex vivo to understand the in vivo physiological role of non-enzymatic activation of C5. We hoped to answer whether C5b-like, which existence in vivo has never been proved, was generated in the TME or not.C5 faced a conformational change when serum was acidified with HCl or lactic acid. In a pro-oxidative environment with serum-supplemented with H2O2, a low amount of C5b-like was detected, but by adding iron catalyzer FeCl2, this amount was increased. With adding gallic acid as an antioxidant, the C5 conformational change was increased even further. Hence, C5 non-enzymatic activation in the presence ofH2O2 was iron-dependent, and by adding antioxidants, the turnover of C5b-like increased. In addition, C5b-like was also detected when activating granulocytes in serum. The C5 conformational change was also investigated in conditioned medium from the tumor cell line BXPC-3, an approach to mimic the TME with low pH and pro-oxidative status. BXPC-3 cells produced lactate in cell culture media and C5blikewas produced in the media by adding the serum as a C5 source. In conclusion, we show that a C5 conformational change occurred in low pH, in a prooxidative, and in a pro-inflammatory environment, which are the three characteristics of the TME. Complement activation in the TME may therefore progress without the controlled formation of C5-convertases.