An allergy is an abnormal immune system response to a substance that is typically harmless to most people. This exaggerated immune reaction occurs when the body mistakenly identifies the allergen as a threat and releases chemicals, such as histamines, to defend against it. Common allergens include pollen, certain foods, insect venom, pet dander, and certain medications.

Symptoms of allergies can manifest in various ways, affecting different parts of the body. Respiratory symptoms may include sneezing, nasal congestion, coughing, and shortness of breath. Skin reactions can range from mild redness and itching to more severe conditions like hives or eczema. Eye symptoms may involve redness, itching, and watering. Gastrointestinal symptoms can include nausea, vomiting, abdominal pain, and diarrhea.

Systemic reactions, known as anaphylaxis, are severe and potentially life-threatening allergic responses. Symptoms of anaphylaxis include difficulty breathing, swelling of the face and throat, a rapid or weak pulse, and a drop in blood pressure. Anaphylaxis requires immediate medical attention, as it can lead to a medical emergency.