An allergic reaction occurs when the body's immune system mistakenly identifies a harmless substance, known as an allergen, as a threat. Common allergens include pollen, pet dander, certain foods, and insect stings. When exposed to an allergen, the immune system releases chemicals like histamine, leading to symptoms such as itching, swelling, and difficulty breathing. Symptoms of an allergic reaction can vary in severity. Mild reactions may cause sneezing or a rash, while severe reactions, known as anaphylaxis, can be life-threatening and require immediate medical attention. Treatment often involves avoiding allergens and using medications like antihistamines or epinephrine for severe cases.