Antigen Polarity in B Cell Differentiation Communication received through cell contact is critical for the differentiation of specialized effector cell populations during the immune response. For example, B lymphocytes acquire antigen that they present to helper T lymphocytes. T lymphocytes, in turn, provide key differentiation signals to B lymphocytes. In order to learn more about this process, Thaunat et al. (p. 475 ; see the Perspective by Dustin and Meyer-Hermann ) used multiphoton microscopy and imaging flow cytometry to visualize the localization of antigen in B lymphocytes during an immune response. Antigen acquired by B lymphocytes exhibited a polarized distribution that was sustained over several rounds of cell division. This produced a population of activated B lymphocytes that contained very low levels of antigen. Daughter cells that received more antigen were better able to stimulate T cells. Because cues received through T lymphocyte interactions are likely to influence B lymphocyte fate decisions, unequal distribution of antigen in dividing B lymphocytes may influence their differentiation.