Describe a symptom that could indicate an allergic reaction to a cosmetic product.

Redness or swelling of the skin is a classic symptom of an allergic reaction to a cosmetic product. When the skin comes into contact with a substance that triggers an allergic response, the body reacts by releasing histamines, which lead to inflammation. This inflammation often manifests as redness or swelling in the affected area. It is essential for cosmetologists and skincare professionals to recognize these symptoms, as they may indicate that the product contains ingredients that the individual cannot tolerate, necessitating an immediate response and a reassessment of the products being used.

Other potential symptoms, such as itching and dryness, can also occur but are not as definitive as redness and swelling when it comes to indicating an underlying allergic reaction. Increased shine on the skin or improvement in skin texture are more indicative of positive responses to certain products rather than reactions, and so they would not be considered signs of an allergic reaction.