| Location | Peripheral tissues, central nervous system | Gastric parietal cells, cardiac muscle, mast cells |
| Signal Transduction | Gq protein-coupled receptor | Gs protein-coupled receptor |
| Activation | Histamine binding leads to increased intracellular calcium and activation of phospholipase C pathway | Histamine binding leads to increased intracellular cAMP and activation of protein kinase A pathway |
| Function | Mediates allergic reactions, smooth muscle contraction, vasodilation, increased vascular permeability, itching | Regulates gastric acid secretion, cardiac muscle contractility, mast cell degranulation |
