Factor VIII-related antigen (FVIII) is the von Willebrand factor, a large protein, built up by more than 2000 aminoacids. This product of the von Willebrand factor gene is tissue-specific and present only in endothelial cells, megakaryocytes and thrombocytes. In the endothelial cells, a granular pattern of reactivity may be seen. Endothelial cells of large blood vessels, liver sinusoids and lymphatics are often FVIII negative. FVIII is insuffiently produced in patients with the inherited von Willebrand disease.
FVIII is expressed in most tumours with endothelial differentiation. In many tumours, it is constantly expressed, while in others, such as angiosarcoma, Kaposi sarcoma and Dabska tumour, the antigen can be demonstrated in a majority of, but not not all, cases. FVIII has occasionally been demonstrated in non-vascular tumours, such as myxoma, chemodectoma, and breast carcinoma. In glomus tumours, FVIII is generally not possible to demonstrate. Since von Willebrand factor is produced in megakaryocytes, the rare cases of megakaryocytic leukaemia also are positive in most cases.
Due to the specificity of the antigen, rather than the sensitivity, FVIII is often a part of the panel for the identification of benign and malignant vascular neoplasms, along with CD31 and CD34.
Normal appendix is useful. The endothelial cells in all types af vessels should be labelled.
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