The CD15 (HI98) antibody recognizes a 220 kDa carbohydrate antigen–Lacto–N–fucopentaose III, also called Lewis X, X-hapten, and SSEA-I. CD15 antigen is expressed on mature granulocytes (strongly), monocytes (weakly), immature bone marrow cells of myelomonocytic lineage, peripheral blood T lymphocytes (weakly), and some T-cells. CD15 antigen is also expressed on leukemia cells of myelomonocytic origin, Langerhans cells, a variety of carcinoma cells (preferentially adenocarcinomas), and occasionally on lymphocytic leukemia cells. CD15 is absent from B lymphocytes, erythrocytes and platelets. A soluble form of CD15 exists in serum (plasma) besides a membrane form. CD15 antigen plays a role in mediating phagocytosis, bactericidal activity and chemotaxis.