Antihistamines are medications used to treat allergic reactions. When applied to the skin, antihistamines in topical form can help relieve itching, redness, and swelling caused by allergic reactions, insect bites, or other skin irritations. They work by blocking histamine, a substance produced by the body during an allergic response, from binding to its receptors on cells. This prevents histamine from causing symptoms like itching and inflammation, providing relief to the affected area.