HRH4 and cancer: A plethora of studies have related the heterogenic and complex pharmacology of histamine receptors to various diseases: H1R to the allergic inflammation, anaphylaxis, and motion sickness [28,29], H2R to the stimulation of gastric acid secretion leading to peptic ulcer, GERD and aspiration pneumonitis [30,31], H3R to the neurotransmission controlling sleep, cognitive processes, schizophrenia, epilepsy, and pain [32,33,34,35,36,37], and H4R to the immune responses (cancers, myocarditis) and inflammation [38,39,40,41,42] (Figure 2).