Which symptom might suggest a serious allergic reaction?

Swelling of the lips is a significant symptom that can indicate a serious allergic reaction, commonly known as anaphylaxis. This condition occurs when the body reacts excessively to an allergen, and swelling in areas such as the lips can suggest that the reaction is severe. This type of swelling typically results from the release of histamines in the body, which can cause inflammation and constriction of airways, making it a critical sign that immediate medical attention is required. Recognizing such symptoms is essential for timely intervention and management of severe allergic reactions.

In contrast, while a headache, fatigue, or pale skin may indicate a variety of health issues, including mild allergic reactions or other conditions, they are not as specific or critical warning signs as lip swelling when considering the immediate risks associated with allergies.