The epithelial component in type B1 thymomas is diffusely positive for CK19, p63, and PAX8. They are focally positive for CK7, CK8, CK14, and CK18. They are negative for CK20. The cortical areas are composed predominantly of a dense infiltrate of immature T-cells which are TdT+, CD1a+, CD3+, CD4+, CD8+, and CD34-ve. The medullary islands contain mature T-cells which are CD3+, TdT-ve, CD1a-ve, and either CD4+ or CD8+. There are also mature B-cells which express CD20 and CD79a. The epithelial cells in medullary islands are diffusely positive for CK19. The image shows scattered neoplastic thymic epithelial cells surrounded by non-neoplastic lymphocytes. The epithelial cells have pale oval or round nuclei with inconspicuous nucleoli.