Recent data show that B cells and plasma cells located in tumours or in tumour-draining lymph nodes can have important roles in shaping antitumour immune responses. In tumour-associated tertiary lymphoid structures, T cells and B cells interact and undergo cooperative selection, specialization and clonal expansion. Importantly, B cells can present cognate tumour-derived antigens to T cells, with the functional consequences of such interactions being shaped by the B cell phenotype. Furthermore, the isotype and specificity of the antibodies produced by plasma cells can drive distinct immune responses. Here we summarize our current knowledge of the roles of B cells and antibodies in the tumour microenvironment. Moreover, we discuss the potential of using immunoglobulin repertoires as a source of tumour-specific receptors for immunotherapy or as biomarkers to predict the efficacy of immunotherapeutic interventions.