B lymphocytes are cells of the lymphoid lineage that express antibody as their surface antigen receptor (BCR). The principle purpose of B cell activation by antigen is to induce their differentiation into antibody-producing plasma cells, via a germinal centre reaction (Fig. 1). In addition, B cells act as important antigen-presenting cells, activating CD4T cells and may also produce immunoregulatory cytokines such as interleukin (IL)IL10. B cells influence allograft outcomes in transplantation via antibody production (HLA and non-HLA antibodies) contributing to acute and chronic antibody-mediated rejection (ABMR), may drive T cell mediated rejection (TCMR) via antigen presentation, and influence the development of transplant tolerance via IL10 production [1Clatworthy M.R., Targeting B. cells and antibody in transplantation Am J Transplant 2011 ;  11 : 1359-1367 [cross-ref]

Cliquez ici pour aller à la section Références]. A better understanding of basic B cell biology is required to develop therapeutic strategies that target B cells, allowing improved control of humoral responses in transplantation; Data show that the B cell activation threshold is determined by BCR affinity for antigen and by positive and negative co-signals, including those provided by CD4T cells, such as co-stimulation via CD40-CD40L interactions and via cytokines such as IL4 and IL21 and those produced by myeloid cells, for example, BAFF [2Goodnow C.C., Vinuesa C.G., Randall K.L., Mackay F., Brink R. Control systems and decision making for antibody production Nat Immunol 2010 ;  11 : 681-688 [cross-ref]

Cliquez ici pour aller à la section Références]. High serum levels of BAFF have been associated with an increased risk of developing de novo donor-specific antibodies and of ABMR in sensitised subjects [3Thibault-Espitia A., Foucher Y., Danger R., Migone T., Pallier A., Castagnet S., et al. BAFF and BAFF-R levels are associated with risk of long-term kidney graft dysfunction and development of donor-specific antibodies Am J Transplant 2012 ;  12 : 2754-2762 [cross-ref]

Cliquez ici pour aller à la section Références, 4Banham G., Prezzi D., Harford S., Taylor C.J., Hamer R., Higgins R., et al. Elevated pre-transplant soluble BAFF is associated with an increased risk of acute antibody-mediated rejection Transplantation 2013 ;  96 : 413-420 [cross-ref]

Cliquez ici pour aller à la section Références]. A small proportion of plasma cells arising from the germinal centre become established as long-lived plasma cells in the bone marrow. They reside within a number of limited niches, do not proliferate, but act as long-term antibody factories, producing IgG. Plasma cells have also been described in inflamed tissues in autoimmunity and within allografts [5Thaunat O., Field A.C., Dai J., Louedec L., Patey N., Bloch M.F., et al. Lymphoid neogenesis in chronic rejection: evidence for a local humoral alloimmune response Proc Natl Acad Sci U S A 2005 ;  102 : 14723-14728 [cross-ref]

Cliquez ici pour aller à la section Références]. A number of these processes may be amenable to therapeutic targeting, and this will be discussed [6Clatworthy M.R. B-cell regulation and its application to transplantation Transplant international 2014 ;  27 : 117-128 [cross-ref]

Cliquez ici pour aller à la section Références].