Allergies are hypersensitive immune responses that occur when the body reacts to normally harmless substances—known as allergens—such as pollen, dust, food, insect venom, or medications. In people with allergies, the immune system mistakenly identifies these substances as threats and releases chemicals like histamine, leading to symptoms such as sneezing, itching, watery eyes, skin rashes, or more severe reactions like asthma or anaphylaxis.