- IgG4-related disease (IgG4-RD) is a potentially multi-organ disease with highly characteristic pathology findings and immunostaining characteristics across involved tissues.
- Organ system involvement may be confined to single organs but in many cases evolves over months to years to involve multiple organs in either a sequential or simultaneous fashion.
- Commonly involved organs include the salivary glands (submandibular, parotid); the orbits and lacrimal glands; the thyroid gland; the lymph nodes; the thoracic and abdominal aorta; the mediastinum, retroperitoneum, and mesentery; the lungs, biliary tree, pancreas, and kidneys.
- IgG4-RD has also been reported in the pachymeninges, the skin, and the prostate gland.
- Serum IgG4 concentrations are elevated in most patients (approximately 70%). One explanation for the finding of normal serum IgG4 concentrations may be the prozone phenomenon, a spuriously low result reported sometimes when the analyte (IgG4) is actually present at exceptionally high concentrations. This problem may be circumvented by diluting test samples sufficiently.
- Serum IgG4 concentrations sometimes correlate with disease activity.
- Histopathologic hallmarks: lymphoplasmacytic tissue infiltrate, storiform fibrosis, obliterative phlebitis, germinal center formation, and mild to modest tissue eosinophilia.
- Immunostaining characteristics: A high percentage of plasma cells stain positively for IgG4.
IgG4-RD is a systemic fibroinflammatory condition recognized in the first decade of this century and now identified increasingly across a wide array of organ systems. The condition is characterized by a tendency to form tumefactive lesions; a dense lymphoplasmacytic infiltrate rich in IgG4-positive plasma cells; storiform fibrosis; and, often but not always, elevated serum IgG4 concentrations. The first organ within the spectrum of IgG4-RD to be linked with elevations in serum IgG4 concentrations was the pancreas. Pancreatic involvement by IgG4-RD is now termed type 1 autoimmune pancreatitis. In 2003, extrapancreatic manifestations were identified in patients with this pancreatic disease, and IgG4-RD has now been described in virtually every organ system: the biliary tree, salivary glands, periorbital tissues, kidneys, lungs, lymph nodes, meninges, aorta, breast, prostate, thyroid, pericardium, and skin. The histopathologic features bear striking similarities across organs. In 2011, recommendations for the nomenclature of individual organ system manifestations were put forth following an international symposium on this condition.
One consequence of the recognition of IgG4-RD is that many medical conditions once viewed as separate conditions isolated to single organs are now acknowledged to be part of the IgG4-RD spectrum. Examples of this include “Mikulicz syndrome,” “Küttner tumor,” and Riedel thyroiditis. In addition, IgG4-RD also accounts for substantial percentages of diseases characterized by the presence of pseudotumors or fibrotic lesions of previously unclear etiologies. IgG4-RD is responsible for significant proportions of cases of orbital pseudotumors, retroperitoneal fibrosis, and sclerosing mesenteritis, among other cases of inflammation within organs that until recently has been considered to be of obscure origin.
The etiology of IgG4-RD remains unknown. There is no definitive evidence for a link with autoimmunity. Many patients have overlaps with allergic conditions, such as asthma and allergic rhinitis. Few population-based studies of IgG4-RD have been performed and the disease epidemiology remains ...