Why are doctors recommending zantac (ranitidine) for an allergic reaction or hives?

Good question: Hives (or urticaria) represent a sensitivity to something dilating the small vessels under the skin, causing a raised pink lesion; if u press on it, it will blanch, let it go it pinks up again. Many causes: foods, chemicals, perfumes, very often stress. Stress can increase acid formation in stomach, Zantac (ranitidine) reduces it. Over 50% of time, one can't find cause. I treat initially c antihistamines.

Antihistamine: Hives are typically, we think, a histamine driven reaction. There are several different type of histamine receptor. Zantac (ranitidine) is a medicine that blocks the stomach histamine receptor that produces acid. Hence its effectiveness in acid reflux disease. In some patients blocking both the skin histamine receptor and the stomach histamine receptor helps with their hives.

Antihistamine also: Zantac is an antihistamine primarily effective for the histamine receptors in your stomach. They're called h-2 receptors. The histamine receptors in your blood vessels & nerve endings are primarily h-1 receptors with some h-2 receptors. When h-1 active anthistamines like Benadryl (diphenhydramine) or Allegra are not sufficient to control hives Zantac can be added for a little extra control. Alone it's not very good.