The immunologic effects of histamine are exerted via the H1 and H2 histamine receptors (H1R and H2R). The effects of histamine include autoinhibition of histamine release from mast cells and basophils, immunosuppression, and the regulation of the activities of cell-mediated immunity (Th1) and humoral immunity (Th2) cells. The asthma phenotype is associated with relative underexpression of Th1 activities and overexpression of Th2 activities. Although the expression of H1R by Th1 cells is greater than the expression of H2R, the actual role of histamine in maintaining a physiological Th1/Th2 ratio and in regulating the expression of the asthma phenotype has not yet been established.