The process may involve pancreas, bile ducts, gallbladder, liver, thyroid, salivary and lacrimal glands, orbits, lungs, mediastinum, pericardium, aortae and arteries, retroperitoneum, kidneys, prostate and testes, breast, lymph nodes, skin, hypophysis etc. Clinical manifestations, actually regarded in the setting IgG4-RD, include broad variety of conditions [3] [4] [5] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] . Abstract Immunoglobulin G4-related

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... isease (IgG4-RD) is a new entity, which comprise a group of conditions, sharing common clinical, serologic and pathologic features -mainly tumor-like swelling of involved organs, lymphoplasmacytic tissue infiltration with the predominance of IgG4 positive plasma cells and CD4 positive T lymphocytes, modest tissue eosinophilia, and so-called "storiform" fibrosis with cartwheel appearance of the arranged fibroblasts and inflammatory cells. The pathogenesis of IgG4-RD is not fully understood, and the pathogenic role of IgG4 is still under discussion. The pathology process may involve pancreas, bile ducts, gallbladder, liver, thyroid, salivary and lacrimal glands, orbits, lungs, mediastinum, pericardium, aortae and arteries, kidneys, prostate and testes, breast, lymph nodes, skin, hypophysis etc., but one of the most common manifestations is retroperitoneal fibrosis. Several case series data showed that IgG4-RD is responsible for a majority of cases of retroperitoneal fibrosis, previously regarded as "idiopathic". The diagnosis of IgG4-RD requires pathology data; lymphoplasmacytic tissue infiltration with mainly IgG4+ plasma cells and lymphocytes is confirmatory. Serum IgG4 levels should be measured and isolated IgG4 elevation is a significant aid in diagnosis, although it is not diagnostic. Elevated IgG4 in patients with IgG4-RD were shown to be usually polyclonal First-line treatment for symptomatic IgG4-RD is prednisone. Here we present a pathology proven case of IgG4-RD, manifested as retroperitoneal fibrosis and monoclonal gammopathy, successfully treated with corticosteroids.