HRH4 and breast cancer: Besides the well-documented immune cell responses of histamine, which are mediated by H1R and H2R, new physio- and pathophysiological roles of the latest discovered H4R have been reported over the last decade.12–16 The H4R is mainly expressed in cells of the immune system, such as mast cells, basophils, eosinophils, monocytes, dendritic cells, T lymphocytes and natural killer (NK) cells,12,15 and its functional expression is demonstrated in different types of tumours.11,17–20 H4R was found in human breast cancer tissues and cell lines.