Immunoglobulin G4 (IgG4)-related disease (IgG4-RD) is a rare multisystem inflammatory disease of unknown cause resulting in a collection of disorders that share pathologic, serologic, and clinical features. IgG4+ plasma cells infiltrate involved organs. Secondary organ fibrosis and accompanying high levels of circulating IgG4 are often seen. Recorded cases occur more commonly in middle- and advanced-aged individuals, and the condition has a slight male predominance. Pediatric cases of systemic IgG4-RD are noted in the literature, but few cases note cutaneous involvement.

Clinical manifestations are variable depending on the organ system involved. Most commonly, IgG4-RD is associated with autoimmune pancreatitis (see chronic pancreatitis), a rare, chronic disease with varied presentation, often mimicking pancreatic malignancy, making diagnosis challenging. Autoantibodies in the pancreas cause inflammation and enlargement. Other manifestations include lymphadenopathy, biliary obstruction (may present similarly to primary sclerosing cholangitis), membranous nephropathy, salivary dysfunction, lacrimal gland dysfunction, retroperitoneal fibrosis, coronary arteritis, cutaneous plaques on the head and neck, and atopy (asthma, eczema), etc, although nearly any organ system can be affected. Symptoms typically develop over months to years, and treatment success is variable.

Proposed diagnostic criteria include:

1. Clinical examination shows characteristic diffuse / localized swelling or masses in single or multiple organs.
2. Hematological examination shows elevated serum IgG4 concentrations (135 mg/dL).
3. Histopathologic examination shows:
• Marked lymphocyte and plasmacyte infiltration and fibrosis.
• Infiltration of IgG4+ plasma cells: ratio of IgG4+ / IgG+ cells > 40% and > 10 IgG4+ plasma cells per high-power field.

The diagnosis is definite if all 3 criteria are present. If the first and third criteria are present, the diagnosis is probable. If the first and second criteria are present, the diagnosis is possible.