How does ranitidine work?

Ranitidine (Zantac) is an H2 blocker. Histamine is released by nerves to the stomach and bind to H2 receptors to signal the stomach to make acid. Ranitidine blocks this interactions and so decreases acid production.

The drugs called "antihistamines" block the H1 histamine receptors so that histamine (the same chemical) released by immune cells can't interact with other immune cells to cause Allergies.