An antihistamine is a type of pharmaceutical drug that opposes the activity of histamine receptors in the body. Antihistamines are subclassified according to the histamine receptor that they act upon: the two largest classes of antihistamines are H₁-antihistamines and H₂-antihistamines. Antihistamines that target the histamine H₁-receptor are used to treat allergic reactions in the nose (e.g., itching, runny nose, and sneezing) as well as for insomnia. They are sometimes also used to treat motion sickness or vertigo caused by problems with the inner ear. Antihistamines that target the histamine H₂-receptor are used to treat gastric acid conditions (e.g., peptic ulcers and acid reflux). H₁-antihistamines work by binding to histamine H₁ receptors in mast cells, smooth muscle, and endothelium in the body as well as in the tuberomammillary nucleus in the brain; H₂-antihistamines bind to histamine H₂ receptors in the upper gastrointestinal tract, primarily in the stomach.