HRH4 and Alzheimer disease: Consistent with the mRNA results, histamine and the H2R agonist amthamine, but not the H4R agonist ST-1006, induced an upregulation of IL-18Rα at the protein level in eosinophils from AD patients, while histamine and ST-1006 induced a slight upregulation in cells from healthy controls (Figure 3b,c).