**Abstract**

Immunoglobulin G4 (IgG4)-related disease (IgG4-RD) is an immune-mediated condition that can affect almost any organ. It is a chronic, systemic, inflammatory condition of unknown etiology. Pseudotumor formation is the most common and characteristic clinical symptom. The variable organ dysfunction reflects the clinical presentation. Because there are not specific antibodies for this disease, histopathological assessment provide the pivotal role in the diagnosis. IgG4-RD is characterized by a lymphoplasmacytic infiltrate composed of IgG4+ plasma cells, storiform fibrosis, obliterative phlebitis and mild to moderate eosinophilia. In this chapter we present the newest knowledge of the IgG4-RD pathogenesis and then concentrate on clinical symptoms which can mimic many other conditions in rheumatology, e.g., this common as Sjӧgren syndrome or rare as vasculitis or idiopathic retroperitoneal fibrosis.

**Keywords:** IgG4-related disease, Mikulicz syndrome, Kuttner's disease, pseudotumor
