Provided are compositions and methods for the treatment of hematological conditions, in particular CD99+ acute myelogenous leukemias (AML) and myelodysplastic syndromes (MDS), which comprise one or more antibody that (a) binds to the extracellular domain of CD99, (b) ligates AML and/or MDS cell-surface expressed CD99, (c) promotes the capping/clustering/aggregation AML and/or MDS cell-surface expressed CD99, and (d) induces apoptosis in and consequent cytotoxicity of antibody-ligated CD99+ AML and/or MDS cells. Disclosed methods include methods for identifying AML and MDS patients that are susceptible to treatment with an anti-CD99 antibody by detecting the elevated expression of CD99 in a tissue sample or cell from an AML or MDS patient and for treating an AML and/or MDS patient exhibiting elevated CD99 gene and or cell-surface protein expression by administering a composition comprising an anti-CD99 antibody, either alone or in combination with one or more additional component such as a mobilizing agent, a transmigration blocking agent, and an AML and/or MDS chemotherapeutic agent, such as daunorubicin, idarubicin, cytarabine, 5-azacytidine, and decitabine.
