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.Linvention concerne des compositions et des méthodes de traitement daffections hématologiques, en particulier des leucémies aigües myéloblastiques (LAM) CD99+ et des syndromes myélodysplasiques (SMD), ces compositions contenant au moins un anticorps qui (a) se lie au domaine extracellulaire de CD99, (b) ligature CD99 exprimé par la surface de cellules LAM et/SMD, (c) favorise le coiffage/regroupement/agrégation de CD99 exprimé par la surface de cellules LAM et/ou SMD, et (d) induit lapoptose et une cytotoxicité consécutive des cellules LAM et/ou SMD à CD99+ lié à lanticorps. Les méthodes selon linvention consistent, entre autres, à identifier les patients atteints de LAM et SMD, qui sont sensibles à un traitement par un anticorps anti-CD99, par détection dune expression élevée de CD99 dans un échantillon tissula
