Human immunoglobulin preparations are used therapeutically for various disorders. Such therapy is generally safe but adverse effects occasionally occur in recipients. It has been suggested that antibodies to cytokines present in clinical immunoglobulin products may contribute to undesirable effects in recipients. Therefore, we investigated intravenous and intramuscular immunoglobulin products for the presence of cytokine‐specific neutralizing antibodies. Using validated bioassays, we detected neutralizing activity against human granulocyte–macrophage colony‐stimulating factor (GM‐CSF), interferon‐α2a (IFN‐α2a) and interleukin‐1α (IL‐1α) in immunoglobulin products. We found no neutralization of granulocyte colony‐stimulating factor, macrophage colony‐stimulating factor, stem cell factor, IL‐1β, IL‐2, IL‐3, IL‐4, IL‐6, IL‐9, IL‐10, IL‐12, tumour necrosis factor‐α, oncostatin M (OSM) and IFN‐γ. Most batches which neutralized IFN‐α2a activity also neutralized other IFN‐α subtypes, IFN‐ω and IFN‐β. Most products (94%) neutralized the biological activity of GM‐CSF. No correlation between batches and their ability to neutralize bioactivities of GM‐CSF, IFN‐α2a and IL‐1α was found. This neutralizing activity could be traced to plasma pools used for manufacture of immunoglobulins. The neutralization was mediated by specific cytokine antibodies contained within immunoglobulin products as it was present in specific immunoglobulin G (IgG) fractions eluted from cytokine affinity chromatography columns. Specific binding of such IgG fractions to cytokines in immunoblots and in enzyme‐linked immunosorbent assays (ELISAs) was observed. This contrasts with the broad non‐specific recognition of cytokine proteins observed using unfractionated immunoglobulins in ELISAs. This is the first comprehensive study showing the presence of neutralizing antibodies against GM‐CSF, IL‐1α, or IFN‐α2a in immunoglobulin products
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