CD15 is a complex cluster of cell surface glycoproteins and glycolipids having a common terminal pentasaccaharide known as the Lewisx (Lex) antigen. CD15 is a haemopoietic differentiation antigen expressed on most terminally differentiated myeloid cells including granulocytes, eosinophils, mast cells, monocytes/macrophages, and Langerhans' cells. CD15 is not substantially expressed on haemopoietic progenitor cells. The positivity for CD15 is characteristic of Hodgkin’s cells in classical Hodgkin’s disease (HD). Rare cases of acute lymphoblastic leukaemia, in which myeloid antigens are often CD15 positive. Myeloid leukaemia cells express CD15 in a heterogeneous manner. CMLs are regularly CD15 positive. CD15 is expressed in a varying proportion of epithelial tumours such as adenocarcinomas (particularly from breast, lung and colon), renal cell carcinoma, apocrine carcinoma of the skin, papillary and follicular carcinoma of the thyroid, and serous carcinoma of the ovary. It is possible that sialyl-CD15 confer on the tumour cells the capacity to metastasize. Malignant mesothelioma is practically always CD15 negative (positivity has been reported in up to 6%, particularly the desmoplastic variant). In gliomas, CD15 positivity inversely correlates with the grade of malignancy. Among germ cell tumours, CD15 is detected only in mature teratoma. In haematopathology CD15 is important for the diagnosis of classical HD and characterization of acute leukaemia. CD15 may be used for histopathological grading of gliomas and differentiating between malignant gliomas and non-neoplastic glial cells (the latter usually strongly stained). Kidney and tonsil are recommended as positive and negative tissue controls for CD15.