CD48 (BLAST) is an activation-associated, glycosylphosphatidylinositol (GPI)-anchored cells surface glycoprotein expressed primarily in mitogen-stimulated human lymphocytes. CD48 is found on the surface of lymphocytes and other immune cells, dendritic cells and endothelial cells and participates in activation and differentiation pathways in these cells. CD48 is expressed on T cells, B Cells, thymocytes and splenocytes. Both normal and malignant white blood cells express CD48 on their membrane surface, but greater than 95% of CD34+ hematopoietic stem cells do not express CD48. CD48 is expressed at higher levels on human Burkitt’s lymphoma cell lines, Raji and most acute myeloid leukemia cells with phenotype CD34-/CD13+/CD33+. Although much remains to be elucidated, CD48 is a critical marker for human immunity and will most likely be of use in the treatment of many diseases like hematopoietic tumors, autoimmunity, allergy and chronic inflammatory diseases.