Allergic diseases are a group of conditions caused by hypersensitivity of the immune system to typically harmless substances in the environment, known as allergens. These conditions include allergic rhinitis, asthma, atopic dermatitis, food allergies, and anaphylaxis. Symptoms vary widely depending on the allergen and the affected organ system, ranging from mild discomfort to life-threatening reactions. Diagnosis often involves allergy testing, and management focuses on allergen avoidance, pharmacotherapy, and immunotherapy.