The primary symptom of an allergic reaction is usually itching. Other common symptoms can include hives, swelling, and respiratory issues. Allergies occur when the immune system mistakenly identifies harmless substances, such as pollen or certain foods, as threats. This triggers the release of histamines and other chemicals, leading to the unpleasant symptoms associated with allergies. Severe reactions can result in anaphylaxis, a life-threatening condition requiring immediate attention.