The role of complement system in bridging innate and adaptive immunity has been confirmed in various invasive pathogens. The aim of this study is to investigate the alteration of complement C3 in patients with severe abdominal sepsis and evaluate the role of complement C3 depletion in prognosis of such patients. The relationship between complement C3 depletion and adaptive immunity is studied meanwhile.

Severe abdominal sepsis remains a significant cause of death in patients undergoing intra-abdominal infection, in despite of recent declines in overall mortality. There is a abundant evidence to suggest complement activation during sepsis. While there is great interest in complement by-products in human sepsis, few studies focus on the persistent consumption of complement components and its role in prognosis of sepsis. Complement C3 is indispensable community pathway for complement activation. In a way, the alteration of C3 levels can affect the whole status of complement biological functions.

In clinical practice, the severe abdominal sepsis would develop compromised immune function if the intra-abdominal infection is not well controlled. The down-regulated T- and B-cell immune responses to sepsis are correlated to the decreased immune defense. To our knowledge, there are few human data that have investigated the relationship between complement depletion and adaptive immunity in severe abdominal sepsis. The investigators hypothesize that the complement C3 depletion during sepsis has a stronger association with the down-regulated adaptive immunity and can be regarded as a essential risk factor to predict the prognosis of such critical illness.

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.

Inclusion Criteria:

• Clinical diagnosis of severe abdominal sepsis

Exclusion Criteria:

• Age < 18 or > 60 years
• Pregnancy
• Leucopenia from radiochemical therapy due to malignant tumor
• Any primary diagnosis other than sepsis
• Confirmed immunodeficiency
• Requirement for blood transfusion, plasmapheresis, or immediate surgery