The causes of secondary immune thrombocytopenia (ITP), which account for approximately 18 to 20% of all adult ITP cases, include some viral infections (Cines et al , 2009; Moulis et al , 2014). Indeed, ITP can be triggered by or associated with many viruses including hepatitis C virus, human immunodeficiency virus, cytomegalovirus, Epstein‐Barr virus and others like severe acute respiratory syndrome coronavirus 1 (SARS‐CoV‐1) (Boyer Chammard et al , 2017; Tanaka et al , 2003; Yang et al , 2005; Cines et al , 2009). Among the suspected mechanisms, antibodies directed against virus glycoproteins may cross‐react with platelet surface integrins like GPIIb/IIIa or GPIb‐IX‐V (Assinger, 2014).

In addition to occasional autoimmune hemolytic anemia, unexplained iron-deficiency anemia has been reported in childhood Castleman disease (CD). The recent discovery of hepcidin has regenerated the research on iron metabolism. This hormone is a key regulator of iron homeostasis, mainly by inhibiting intestinal iron absorption. Liver expression of hepcidin increases in response to interleukin 6 (IL-6). With chronic overproduction of IL-6 as a hallmark, CD could be an interesting human model for studying the contribution of the IL-6/hepcidin pathway in the pathogenesis of anemia of chronic disease. We report here the case of a 16-year-old boy with chronic iron-deficiency anemia (plasma ferritin: 19 μg/L; plasma iron: 2.2 μmol/L; negative bone marrow Perls' Prussian blue stain), inflammatory syndrome (C-reactive protein: 108 mg/L), and growth retardation for the previous 2 years. Diagnostic workup revealed a large mesenteric mass corresponding to localized CD of mixed histologic type. Resection of the tumor resulted in complete resolution of iron-deficiency anemia and inflammatory syndrome. Parallel variations of plasma IL-6, C-reactive protein, and hepcidin concentrations, together with tumor immunohistochemistry, strongly suggested that IL-6 synthesized by the tumor caused both the inflammation and iron deficiency through enhancement of hepcidin production by the liver. The results of this unique case study (1) explain the mechanism of iron deficiency observed in some children with CD, (2) confirm in vivo the regulatory effect of IL-6 in human hepcidin production, and (3) suggest that iron deficiency is a causal link between IL-6 and anemia of chronic disease.