Food allergies are a sign that your immune system is a little confused. Depending on what you're allergic to, the proteins in that food scare your body into thinking it's being attacked by a virus or bacteria -- even though the proteins are perfectly safe. In response to you eating these proteins, your immune system sends out an antibody called immunoglobulin E to fight off the allergen. The antibody then triggers a bunch of chemicals to wage war against the protein, including the chemical histamine, which is responsible for most allergic symptoms.
Among these food allergy symptoms are rashes. They can develop minutes or hours after you eat a food you're allergic to. Red, itchy bumps called hives are very common among food allergy sufferers; some people find themselves with eczema, and some just itch. Since the rash is caused by the release of histamine, an over-the-counter antihistamine is usually enough to calm the itching.