C5 is the complement in the blood of the organism and is the first molecule capable of causing damage to MAC. The C5 protein is composed of α and β chains, and the domain of the C5 protein consists of 4 parts with 1669 AA. The complement system can be initiated through three different pathways, among which complement C5 is an indispensable activation product in the body complement system. In either of the three, the result is that C5 has ruptured. C5 lyase cleaves C5 into C5a and C5b, both of which have different roles.

After C5a binds and then activates C5aR, it will cause various effects in the body, such as activating cells and expressing inflammatory factors, and C5a also plays an important role in asthma and inflammatory diseases. Moreover, C5aR1 and C5aR1 also play major roles in Alzheimer's disease and cancer.

The C5b-9 protein in MAC is the final product after the complement system is initiated, C5b-9 protein is formed by multiple proteins, such as C5b with C6, C9, C7, C8, and multiple C 9, and C5b-9 protein appears on the surface of the cell membrane. The final formation of C5b-9 can create the cell membrane pore, by placing itself between the bilayer gaps of the membrane. This causes the body membrane to appear broken, the cells appear dissolved, causing damage to the body. At the same time, C5b-9 is also affected by CD59. If CD59 binds, the production of C5b-9 will be affected, thus protecting the body cells from being broken by C5b-9. In addition, C5b-9 can cause cell death in other ways, causing the body to respond.