['Intravenous immunoglobulin (IVIG) preparations are increasingly used for the treatment of autoimmune and chronic inflammatory diseases. Naturally occurring autoantibodies against Siglec-9 and Fas are thought to contribute to the anti-inflammatory effects of IVIG via cell death regulation of leukocytes and tissue cells. Dimeric IVIG fractions are suspected to contain idiotypic (Id)-anti-idiotypic complexes of antibodies, which might also include anti-Siglec-9 and anti-Fas autoantibodies.', 'Dimeric IVIG fractions were separated from monomeric IVIG by size-exclusion chromatography and remonomerized by low pH treatment. Binding studies of total, monomeric, and dimeric IVIG were performed using surface plasmon resonance and flow cytometry on primary human neutrophils.', 'Anti-Siglec-9 and anti-Fas autoantibodies were contained in both monomeric and dimeric IVIG fractions, but anti-Siglec-9 antibodies were highly enriched in dimeric IVIG. The propensity to engage in dimer formation was paratope dependent. IVIG binding to Siglec-9 was specific and sialylation independent. Interestingly, we detected anti-idiotypic antibodies (anti-Ids) against anti-Siglec-9 autoantibodies in dimeric, but not in monomeric fractions of IVIG.'] Does dimeric IVIG contain natural anti-Siglec-9 autoantibodies and their anti-idiotypes?