The diverse effects of histamine on immune regulation appear to be due to differential expression and regulation of four types of histamine receptors and their distinct intracellular signals. The differences in cellular expression and affinities of these receptors for histamine determine the biological effects of histamine and the drugs that target histamine receptors. In this issue, Dijkstra et al., demonstrate the expression and some of the functions of histamine H(4) receptors on inflammatory dendritic cells in atopic dermatitis skin.