The who, how and where of antigen presentation to B cells.

A functional immune system depends on the appropriate activation of lymphocytes following antigen encounter. In this Review, we summarize studies that have used high-resolution imaging approaches to visualize antigen presentation to B cells in secondary lymphoid organs. These studies illustrate that encounters of B cells with antigen in these organs can be facilitated by diffusion of the antigen or by the presentation of antigen by macrophages, dendritic cells and follicular dendritic cells. We describe cell-surface molecules that might be important in mediating antigen presentation to B cells and also highlight the key role of B cells themselves in antigen transport. Data obtained from the studies discussed here highlight the predominance, importance and variety of the cell-mediated processes that are involved in presenting antigen to B cells in vivo.