C3 plays a pivotal role in initiating both the classical and alternative pathways of the complement system. Upon activation by C3 convertases, C3 is cleaved into C3a and C3b fragments. C3b covalently binds to cell surface carbohydrates or immune aggregates, while C3a acts as an anaphylatoxin mediating local inflammatory responses. Beyond inflammation, C3 regulates the intensity and direction of immune responses through interactions with other complement components. Detection of C3 levels is valuable for diagnosing, monitoring, and assessing prognosis in various diseases, such as autoimmune disorders, infectious diseases, and cardiovascular conditions. As a crucial component of the complement system, C3 is a significant target for drug development and immunotherapy research.