Table 1.

Types and functions of different histamine receptors.

	Expression in Cell Types	Function	Available studies in relation to COVID-19
Histamine 1 Receptor (H1R)	neurons, endothelial cells, adrenal medulla, muscle cells, hepatocytes, chondrocytes, monocytes, neutrophils, eosinophils, dendritic cells (DCs), T cells, and B cells	•activation of Th1 lymphocytes, and decreased humoral immunity	•none
Histamine 2 Receptor (H2R)	parietal cells of the gastric mucosa, muscle, epithelial, endothelial, neuronal, hepatocyte, and immune cells	•antagonizes some of the effects mediated by H1R and leads to the relaxation of smooth muscle cells, causing vasodilation. •inhibition of CXCL10, IL-12, and TNF-α stimulation of IL-10, which is likely associated with Th2 polarization	•Observational studies [[3], [4], [5]] •Multi-site Adaptive Trials [6]
Histamine 3 Receptor (H3R)	identified in the central nervous system and peripheral and presynaptic receptors	•control the release of histamine and other neurotransmitters	•none
Histamine 4 Receptor (H4R)	preferentially expressed in the intestine, spleen, thymus, bone marrow, peripheral hematopoietic cells, and cells of the innate and adaptive immune systems.	•Activation causes chemotaxis in mast cells and eosinophils, leading to accumulation of inflammatory cells and control of cytokine secretion •increased secretion of IL-31 by Th2 cells	•none