IgG4‐related sclerosing disease is a syndrome characterised clinically by mass lesions which commonly involve exocrine glands, elevated serum IgG4 titre, frequent presence of circulating auto‐antibodies, and favourable clinical response to steroid therapy, and pathologically by lymphoplasmacytic infiltration, sclerosis, phlebitis and increased IgG4 + plasma cells. The histologic features can be broadly categorised into three patterns: pseudolymphomatous (Pattern I), mixed (Pattern II) and sclerosing (Pattern III). These patterns may not represent distinct histologic manifestations, but probably snap‐shots in the morphologic evolution of the disease. Immunohistochemistry plays an important role in confirming the diagnosis: absolute number of IgG4 + plasma cells should be over 50 per high power field in areas with the largest number of positive cells, and the IgG4/IgG percentage should be over 40%.
While the pancreas is the prototypic site of involvement (autoimmune pancreatitis), increasing sites are recognised to be involved by the disease, sometimes quite unexpectedly. Examples include hepatobiliary tree, lacrimal gland, salivary gland, lymph node, prostate, lung, kidney, soft tissue, retroperitoneum, mesentery, aorta, mediastinum, breast, pituitary, meninges, upper aerodigestive tract and skin. Most patients have disease involving two or more sites, in various combinations, with or without pancreatic involvement, although the disease can rarely be confined to one organ‐system.