Immunoglobulin A multiple myeloma presenting with Henoch–Schönlein purpura associated with reduced sialylation of IgA1

Summary. Henoch–Schönlein purpura is characterized by immunoglobulin A1 (IgA1) depositions in blood vessels of the skin or in glomeruli, resulting from altered hinge region O‐glycosylation. Henoch–Schönlein purpura is seldom reported as a complication of IgA1 myeloma, even when the circulating IgA concentration is very high. We report two patients with IgA1 myeloma presenting with Henoch–Schönlein purpura. The O‐glycosylation of these patients' IgA1 was studied. Both patients showed increased binding to peanut agglutinin lectin, suggesting a low degree of sialylation of the hinge region of IgA1 that was confirmed by mass spectrometry. IgA multiple myeloma, secreting IgA1 molecules with decreased sialylation, presenting with a Henoch–Schönlein purpura‐like syndrome was diagnosed.