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.