IgG4 is a type of antibody produced by the immune system to fight infection. It is also an important antibody involved in allergy, though it is not felt the condition arising from it is necessarily an allergic condition.
In IgG4-related disease (IgG4-RD), too much IgG4 is produced. It is characterised by mass lesions and/or strictures and can affect almost any organ, though the most common organs affected are the pancreas and bile ducts (in that order) 59. When it affects the bile ducts it is called IgG4-sclerosing cholangitis (IgG4-SC) or IgG4-associated sclerosing cholangitis (IAC) and can mimic PSC and bile duct cancer 59,60.
It is important to make the diagnosis because the disease responds well to treatment with steroids, unlike most patients with PSC 61.
PSC with raised IgG4 levels
People with PSC can have raised IgG4 levels but may not have IgG4-RD. It is not clear whether IgG4-SC and PSC are separate conditions entirely or if they are variations on the same spectrum. There is the suggestion of an overlap between PSC and IGg4-SC because 9-15% of patients with PSC have increased serum IgG4 levels, and some have responded to steroid treatment, especially when younger 61,64.
While elevated IgG4 levels in PSC do not provide a diagnosis of IgG4-SC, it is recommended that where there is a suspicion of IgG4-SC, attempts should be made to make a confirmatory diagnosis, in line with international consensus guidelines 61. A delay in treating IgG4-SC can lead to poorer outcomes 59.