An isolated antibody, or an antigen binding fragment thereof, in which said isolated antibody or said antigen binding fragment specifically binds to the extracellular domain of native human FcγRIIB with greater affinity than said antibody or said binding fragment a antigen binds to native human FcγRIIA, and in which: i) said antibody is 8B5.3.4 antibody produced by a hybridoma cell line having the ATCC accession number PTA-7610； or ii) said antibody or said antigen binding fragment comprises a heavy chain variable domain having the amino acid sequence of SEQ ID NO: 4 and a light chain variable domain having the amino acid sequence of SEQ ID NO: 3 ； or iii) said antibody or said antigen binding fragment comprises the six CDRs of the antibody produced by the hybridoma clone 8B5.3.4 having the ATCC accession number PTA-7610, said six CDRs having the amino acid sequence of SEQ ID NOs : 5-10, wherein said antibody or said antigen binding fragment binds to the same FcγRIIB epitope as that recognized by the antibody produced by hybridoma clone 8B5.3.4 having the ATCC accession number PTA-7610 and competes with said antibody 8B5.3.4 for binding to native human FcγRIIB, and wherein said antibody or said antigen binding fragment has a Kd dissociation constant (Kdisoc / Kasoc) of less than 5 x 10-9 M as determined by surface plasmon resonance.Un anticuerpo aislado, o un fragmento de unión a antígeno de éste, en el que dicho anticuerpo aislado o dicho fragmento de unión a antígeno se une específicamente al dominio extracelular de FcγRIIB humano nativo con mayor afinidad de la que dicho anticuerpo o dicho fragmento de unión a antígeno se une a FcγRIIA humano nativo, y en el que: i) dicho anticuerpo es anticuerpo 8B5.3.4 producido por una línea celular de hibridoma que tiene el número de acceso ATCC PTA-7610； o ii) dicho anticuerpo o dicho fragmento de unión a antígeno comprende un dominio variable de cadena pesada que tiene la secuencia de aminoácidos de SEQ ID NO: 4 y un domini
