What is a common sign of an allergic reaction to hair dye?

Redness or itching of the scalp is a well-known indicator of an allergic reaction to hair dye. This reaction occurs when the immune system responds to certain chemicals present in the dye, often leading to localized inflammation and discomfort on the scalp. It is crucial for individuals experiencing these symptoms after applying hair dye to seek medical advice, as allergic reactions can escalate and pose more serious health risks.

The other options, such as dryness of the hair, split ends, and frizziness, are typically related to hair health and damage rather than indicating an allergic response. They do not involve an immune reaction and can result from factors like environmental stressors, improper hair care techniques, or the use of harsh products.