Antiperinuclear factor (APF) and antikeratin antibody (AKA) have long been known to be associated with rheumatoid arthritis. Human buccal mucosa epithelial cells have hitherto been required as the substrate in the APF test, while AKAs are detected on rat esophagus sections, using an indirect immunofluorescence technique. These two autoantibodies proved to be interrelated. Cytoplasmic inclusions in buccal cells have presumptively been termed keratohyalin granules and the APF target antigen colocalizes exactly with that of antiprofilaggrin antibody within the perinuclear organelles. The latter protein has convincingly been identified as the genuine specificity of the so-called AKA.
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