Close encounters of the first and second kind: T-DC and T-B interactions in the lymph node.

Cellular interactions in lymphoid organs initiate the immune response and determine its outcome. Using two-photon microscopy in the lymph node, several groups have begun to investigate the motility characteristics and interactions among T lymphocytes, B lymphocytes, and dendritic cells (DC) in lymphoid organs. In the first "close encounter", T cells of a particular antigen specificity interact with antigen-bearing dendritic cells and begin to activate. Activation of both CD4+ and CD8+ T cells evolves through several stages; from transient interactions to stable clusters and later to dissociation and proliferation of T cells (clonal expansion). The second "close encounter" requires that antigen-engaged B cells become accessible to T cells by directed migration to the edge of the follicle. T cells and B cells then pair up and waltz together for an extended period, while helper T cells provide signals for B cells to differentiate into plasma cells. In this topical review, we compare the activation choreography of CD4+ T cells interacting first with dendritic cells, and then with B cells, during initiation of the humoral immune response.