C3 complement is the most abundant complement component in serum, with a large molecular weight, typically around 195,000 units, and is mainly synthesized by the liver, requiring the help of some macrophages. Under the action of C3 convertase, it is generally divided into two fragments, C3A and C3B, and has a role in the classical activation pathway of complement and in the bypass activation pathway. For the normal value of C3 complement, the results are divided into two methods, one is the one-way immunodiffusion method, which has a value of 0.8-1.2 g/L. The other method is the rocket immunoelectrophoresis method, where the normal range is between 0.99-1.46 g/L. If a patient presents with elevated C3 complement, it is important to consider whether the patient has a connective tissue systemic disease, such as the possibility of systemic lupus erythematosus or immune connective tissue system disease. Patients should usually pay attention to regular review and apply hormonal drugs for treatment if necessary.