Anti-neutrophil antibodies of the immunoglobulin M class in autoimmune neutropenia

Anti-neutrophil antibodies of the immunoglobulin G (IgG) class have been implicated in the pathogenesis of autoimmune neutropenia, but few reports have described immunoglobulin M (IgM) anti-neutrophil antibodies. To investigate the prevalence of IgM anti-neutrophil antibodies, sera from 130 patients with possible autoimmune neutropenia were studied for IgG and IgM anti-neutrophil antibodies using an immunofluorescence flow cytometric assay. Twenty-five patients (19%) had IgG anti-neutrophil antibodies exclusively, 21 patients (16%) had both IgG and IgM anti-neutrophil antibodies, and 11 patients (8%) had IgM anti-neutrophil antibodies exclusively. Immunoglobulin M anti-neutrophil antibodies were found in adults and children with isolated chronic neutropenia and in patients with Felty's syndrome, systemic lupus erythematosus, immune thrombocytopenic purpura, and human immunodeficiency virus. Patients with neutropenia with only IgM anti-neutrophil antibodies comprised almost 20% of antibody-positive patients in this study.