B cells as initial sensors of the adaptive immune response

The encounter and acquisition of specific antigen by naïve B cells is the initial stage of the humoral immune response against pathogens. But, until recently, little was known as to how, where and when naïve B cells find antigen in vivo. The development of experimental models that allow antigen tracking in vivo, in concert with confocal and multiphoton imaging technology, has provided crucial information to the field. Several reports published in the last year show initial antigen encounter by naïve B cells in draining lymph node follicles; all of them point to the follicle boundary with the subcapsular sinus (SCS) as a key site at which B cell antigen recognition occurs. The macrophage sheet at the SCS has a critical function in antigen capture, transport to the follicle and presentation to follicular B cells. In addition, B cell antigen encounter takes place very shortly after antigen administration, suggesting that B cells are initial sensors in the adaptive immune response. This review focuses on the latest data regarding the early stages of the B cell immune response.