The purpose of this prospective study is two-fold. First, the investigators observe, in a cohort of patients with severe abdominal sepsis, the levels of complement components and percentages of T cell subsets after admission to evaluate the relationship between complement system and adaptive immunity. Second, the investigators also evaluate the application of the C3 related-indexes (C3, C3a, Factor H, DAF, etc.) to patients undergoing severe abdominal sepsis and to develop an alternative model to predict its prognosis.