What is a common sign of a true allergy?

A rash is a common sign of a true allergy because it typically arises as a direct immune response to an allergen. This response involves histamine release and other inflammatory chemicals that can lead to various types of skin reactions, including urticaria (hives), erythema, or even more severe reactions like anaphylaxis that may manifest with skin symptoms.

Rashes can indicate an allergic reaction to foods, medications, or environmental allergens, distinguishing themselves from other gastrointestinal or pain-related symptoms. While abdominal pain, nausea, and headaches can occur in allergic reactions or other medical conditions, they are not classic signs of a true allergy. The presence of a rash is more specific to an allergic response, making it a critical symptom for identifying true allergies in patients.