The classical activation process of the complement system is a cascade enzymatic reaction process in which the activator binds to C1q and then activates complement in sequence to form C3 convertase and C5 convertase. The main process of the classical pathway is a cascade of enzymatic reactions in which the antigen-bound IgG and IgM immune complexes, as the predominant activators, bind to C1q and then sequentially activate C1r, C1s, C4, C2, and C3 to form the C3 convertase, i.e., C4b2a, and the C5 convertase, i.e., C4b2a3b. The specific process is that C1q binds to more than 2 antibody Fc segments and can undergo a conformational change, which activates C1r bound to C1q, and the activated C1r activates the serine protease activity of C1s. C2a binds to C4b to form C3 converting enzyme, and the latter causes C3 to be cleaved to C3a and C3b, which is the pivotal step in the cascade of complement activation reaction. The nascent C3b binds to C4b in C4b2a to form C5 convertase into the terminal pathway of complement activation. The classical activation process of the complement system, as described above, plays an important defense role in the body, helping the body to fight off disease. If you become unwell, you need to go to the hospital as soon as possible.