The road to the discovery of dendritic cells, a tribute to Ralph Steinman.

While it was known by the 1960s that lymphocytes mediated adaptive immunity, it was unknown how antigens stimulated lymphocytes. Between 1967 and 1973, we reported that a rare cell type in murine spleen cells took up antigen and were obligatory for T cell dependent and independent antibody responses. We referred to them as A cells or the third cell type. In 1973, Ralph Steinman and Zanvil Cohn described a rare cell type in murine spleen cells which was phagocytic but had dendrite like protrusions; they named them dendritic cells (DCs). In 1978, Steinman reported that DC were required for mixed lymphocyte reactions. From that time until recent death, Ralph Steinman pursued relentlessly in his laboratory and through collaborations around the world the role and function of DC in immunity. In passing, using a monoclonal antibody supplied by Steinman, we showed that A cells were the same as DC.