Random allergic reactions can be caused by a variety of environmental factors. These can include pollen, dust, pet dander, and certain foods. In some cases, even medications or insect bites can cause an allergic reaction. These reactions occur when the body's immune system mistakes a harmless substance for a dangerous one and releases chemicals to fight it off. Symptoms of an allergic reaction can range from mild discomfort to severe life-threatening conditions. Understanding the potential triggers and how to prevent them is the best way to avoid an allergic reaction.