Nowadays inflammation is commonly defined as a defensive reaction involving the immune system. This paper clarifies the historical roots of this definition. The Russian embryologist Metchnikoff made--100 years ago--the initial observations on phagocytosis that provided a definition of inflammation based on a cellular theory of immunity. Following the many attacks by those who favored the view that immunity was based upon humoral mechanisms, Bordet argued that Metchnikoff's cellular theory of inflammation could be reconciled with the theory of humoral immunity. This study helps the modern reader to grasp the theoretical assumptions supporting our concept of inflammation