Histological Subtypes of AIP
Recently, it has been suggested that AIP be classified into two variants, Type 1 and Type 2 AIP, based on differences in demographics, histology and other organ involvement.[13,14]
The pancreatic histology of Type 1 AIP is known as lympho-plamacytic sclerosing pancreatitis (LPSP), and Type 1 AIP is the variant that has been described and studied more extensively in the literature.[3] Histologicaly, Type 1 AIP is characterized by periductal lympho-plasmacytic infiltrate, rich in IgG4 cells, storiform fibrosis and obliterative phlebitis without GELs. While all these features are usually present in resected specimens, on core needle biopsy only 44% of the core biopsies showed the whole spectrum of changes in one study.[15] It is worth noting that over 15% of pancreatic cancer have IgG4-positive cells without other histologic features of LPSP, and 5-10% of pancreatic cancer subjects have elevation of serum IgG4.[16]
The pancreatic histology of Type 2 AIP is also referred to as IDCP and it is the lesser known AIP variant.[9,17,18] Histological features of Type 2 AIP include predominant neutrophilic infiltrate in the lobule and duct, also known as GELs[9,13,17,18]; the ductal inflammation often leads to duct destruction.[13] The current diagnostic criteria are all geared toward diagnosing Type 1 and not Type 2 AIP. Type 2 AIP does not seem to be associated with an increase in serum IgG4 or other organ involvement, and it is unclear if it responds to steroids.[17,19] Therefore, at present, Type 2 AIP can only be diagnosed histologicaly. Histological diagnosis of both types of AIP requires visualization of the tissue architecture. Hence, a resection specimen or core biopsy is needed for tissue diagnosis.[20,21] Fine-needle aspiration (FNA) is not useful in diagnosing AIP.[21] Core biopsies may potentially be nondiagnostic if the diseases is patchy and the biopsy was obtained from the uninvolved portion. Early data show that, in addition to the histological difference, there may be difference in the age, sex and serology between these two conditions.[13] Thus, AIP may not be one homogenous condition and, by viewing them as two distinct entities, we may have taken an important step towards understanding this condition better. In the remainder of this review, when we use the term AIP alone it connotes Type 1 AIP.
Expert Rev Gastroenterol Hepatol. 2009;3(2):197-204. © 2009 Expert Reviews, Ltd.
Cite this: Autoimmune Pancreatitis: An Update - Medscape - Apr 01, 2009.
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