Expression of MHC class II antigens by placental villi: no relationship with villitis of unknown origin.

The aim of the study was to determine whether immunoreactivity to major histo-compatibility complex (MHC) class II antigens studied by immunohisto-chemistry could be used reliably to define villitis lesions in placenta. Eighteen placental sections with villitis and 32 without, as determined in a careful observer reproducibility study, were immunolabelled with a monoclonal antibody to monomorphic determinants of MHC class II antigens (CR3/43), using a standard avidin-biotin peroxidase technique. Placentas with villitis were found to express MHC class II antigens. However, some showed no immunoreactivity. Occasional villi unaffected by villitis, including those near placental infarcts, also expressed MHC class II antigens. The study therefore showed that immunohistochemistry cannot be used to define villitis of unknown aetiology. It provides further evidence of the difficulties that can arise when immunohistochemistry conflicts with previous light microscopy findings.