Human polyclonal IgG antibodies directly against the nonhuman sialic acid N-glycolylneuraminic acid (Neu5Gc) are potential biomarkers and mechanistic contributors to cancer and other diseases associated with chronic inflammation. Using a sialoglycan microarray, we screened the binding pattern of such antibodies (anti-Neu5Gc IgG) in several samples of clinically approved human IVIG (IgG). These results were used to select an appropriate sample for a multistep affinity purification of the xeno-autoantibody fraction. The sample was then analyzed via our multienzyme digestion procedure followed by nano liquid chromatography (nanoLC) coupled to linear ion trap-Fourier transform mass spectrometry (LTQ-FTMS). We used characteristic and unique peptide sequences to determine the IgG subclass distribution and thus provided direct evidence that all four IgG subclasses can be generated during a xeno-autoantibody immune response to carbohydrate Neu5Gc-antigens. Furthermore, we obtained a significant amount of sequence coverage of both the constant and variable regions. The approach described here, therefore, provides a way to characterize these clinically significant antibodies, helping to understand their origins and significance.