An isolated antibody that specifically binds a human CD73 polypeptide expressed by a cell, wherein the antibody is capable of increasing the proliferation of T cells in the presence of CD39-expressing B cells and ATP, when T cells co-cultured in vitro with CD39-expressing B cells are induced to proliferate by TCR co-stimulation in the presence of ATP, and wherein the antibody comprises: (i) a heavy chain variable domain comprising CDR 1, 2 and 3 of the heavy chain variable region of SEQ ID NO: 3 and (ii) a light chain variable domain comprising CDR 1, 2 and 3 of the light chain variable region of SEQ ID NO: 4; (ii) a heavy chain variable domain comprising CDR 1, 2 and 3 of the heavy chain variable region of SEQ ID NO: 21 and (ii) a light chain variable domain comprising CDR 1, 2 and 3 of the light chain variable region of SEQ ID NO: 22; (iii) a heavy chain variable domain comprising CDR 1, 2 and 3 of the heavy chain variable region of SEQ ID NO: 40 and (ii) a light chain variable domain comprising CDR 1, 2 and 3 of the light chain variable region of SEQ ID NO: 41; (iv) a heavy chain variable domain comprising CDR 1, 2 and 3 of the heavy chain variable region of SEQ ID NO: 42 and (ii) a light chain variable domain comprising CDR 1, 2 and 3 of the light chain variable region of SEQ ID NO: 43; (v) a heavy chain variable domain comprising CDR 1, 2 and 3 of the heavy chain variable region of SEQ ID NO: 44 and (ii) a light chain variable domain comprising CDR 1, 2 and 3 of the light chain variable region of SEQ ID NO: 45; or (vi) a heavy chain variable domain comprising CDR 1, 2 and 3 of the heavy chain variable region of SEQ ID NO: 46 and (ii) a light chain variable domain comprising CDR 1, 2 and 3 of the light chain variable region of SEQ ID NO: 47; wherein the CDR1, 2, and 3 definitions are according to Kabat. (i) a heavy chain variable domain comprising CDR 1, 2 and 3 of the heavy chain variable region of SEQ ID NO: 3 and (ii) a light chain variable domain comprising CDR 1, 2 and 3 of the light chain variable region of SEQ ID NO: 4; (ii) a heavy chain variable domain comprising CDR 1, 2 and 3 of the heavy chain variable region of SEQ ID NO: 21 and (ii) a light chain variable domain comprising CDR 1, 2 and 3 of the light chain variable region of SEQ ID NO: 22; (iii) a heavy chain variable domain comprising CDR 1, 2 and 3 of the heavy chain variable region of SEQ ID NO: 40 and (ii) a light chain variable domain comprising CDR 1, 2 and 3 of the light chain variable region of SEQ ID NO: 41; (iv) a heavy chain variable domain comprising CDR 1, 2 and 3 of the heavy chain variable region of SEQ ID NO: 42 and (ii) a light chain variable domain comprising CDR 1, 2 and 3 of the light chain variable region of SEQ ID NO: 43; (v) a heavy chain variable domain comprising CDR 1, 2 and 3 of the heavy chain variable region of SEQ ID NO: 44 and (ii) a light chain variable domain comprising CDR 1, 2 and 3 of the light chain variable region of SEQ ID NO: 45; or (vi) a heavy chain variable domain comprising CDR 1, 2 and 3 of the heavy chain variable region of SEQ ID NO: 46 and (ii) a light chain variable domain comprising CDR 1, 2 and 3 of the light chain variable region of SEQ ID NO: 47;