What is considered a universal sign of an allergic reaction?

Itching or hives is recognized as a universal sign of an allergic reaction due to the release of histamines and other chemicals in response to an allergen. These substances lead to increased blood flow, causing the skin to become itchy and develop hives, which are raised welts on the skin. This skin manifestation can occur regardless of the specific allergen involved and indicates that the body's immune system is reacting allergically.

Symptoms such as flushing of the skin, shortness of breath, and rapid heartbeat, while they can be associated with allergic reactions, may not universally occur in every situation or for every individual. They are often symptoms of a more severe allergic response, such as anaphylaxis, rather than the typical first indication of an allergic reaction, which commonly presents as itching or hives. This makes itching or hives a more definitive and recognizable sign across various allergic reactions.