Which of the following is a common sign of an allergic reaction?

A common sign of an allergic reaction is watery, itchy, red swollen eyes, which is a symptom of allergic conjunctivitis. Allergic reactions occur when the immune system responds excessively to a harmless substance, known as an allergen, leading to various symptoms. The eyes can react to allergens like pollen, pet dander, or certain foods, causing inflammation and irritation. The presence of itching, redness, and tearing of the eyes is a classic manifestation that individuals experiencing an allergy might report.

In contrast, chest pain is more often associated with cardiovascular issues or respiratory conditions, while nausea without an apparent cause could arise from a multitude of factors, including gastrointestinal issues or other medical conditions. A headache without other symptoms could suggest tension or other types of headache rather than being indicative of an allergic reaction. These options do not specifically represent the hallmark signs of an allergy, which typically include eye symptoms, skin reactions, and respiratory issues.