Coronary artery bypass graft operation (CABG) is a common surgical procedure in Austria and all over the world. Cardiopulmonary Bypass is known to effect a generalized endogenous immune reaction similar to that described in sepsis. This effect may play an important role in development of infectious post-surgical complications, which lead to increased suffering and costs as well as prolonged hospital stay. The production of IL-10 and the involvement of TH2-type lymphocytes seem to be important factors in the development of immune suppression. Although ST2 plays an important role in TH2 effector functions, the protein''s part in immune modulation after CABG has not been studied yet. This book attempts an insight into immune modulation after CABG operation and an explanation of the role of ST2 in this process.