Figure 2.
Immune modulation by histamine produced by various leukocytes. The actions of histamine can be classified into two categories; one mediated by stored histamine and the other by nascent histamine. In the former cases, histamine is produced and stored in the granules of mast cells and basophils and is liberated upon stimulation. In the latter case, HDC is induced at the transcriptional level upon various stimuli, and de novo synthesized histamine modulates the immune responses. Since several functions of histamine have been proposed through the experiments with exogenously-added histamine, the origin of histamine involved in such responses remains to be determined. Recent topics in this field are the identification of the H4R, which is expressed mainly in leukocytes, such as mast cells, eosinophils, and T cells.