Aim. Here, we studied whether the immunoreactivity in pre-operative period defines early postoperative complications in patients with infective endocarditis (IE).Materials and Methods. We consecutively enrolled 110 patients with subacute IE (80 with native-valve IE and 30 with prosthetic valve IE) who underwent a heart valve replacement, then measuring the levels of IgM, IgA, IgG, and complement components in their serum and evaluating their correlation with an adverse early postoperative outcome.Results. Compared with patients< 50 years of age, those ≥ 50 years of age had 2-foldand 3-foldhigher prevalence of systemic inflammatory response syndrome and multiple organ dysfunction syndrome in the early postoperative period. Notably, these numbers correlated with increased С3d and С5а along with reduced Ig М and IgG. IgМ below 9.5 mg/mL and IgG below 2.0mg/mL indicated a 3-foldhigher risk of multiple organ dysfunction syndrome (OR = 3.07, 95% CI = 1.96-4.04, р=0,001).Conclusion. Serum IgM, IgG, and complements factors C3d and C5a may be used as prognostic markers of multiple organ dysfunction syndrome in the early postoperative period.