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

Itching and swelling are classic signs of an allergic reaction, particularly in response to certain chemicals found in hair products like dyes, shampoos, and styling aids. When an individual is allergic to a component in a hair product, the body's immune system reacts by causing inflammation, which results in redness, swelling, and irritation. This can manifest in various areas where the product was applied, such as the scalp, face, or neck, leading to noticeable discomfort and a need to take corrective measures, such as washing out the product or seeking medical attention.

Other symptoms like dryness and flakiness might indicate a separate issue, such as product buildup or sensitivity, rather than a specific allergic reaction. Increased hair growth is typically associated with hormonal changes or certain medications rather than an allergy to hair products. Discoloration of hair can occur from the use of hair dyes or other chemical treatments, but it does not signal an allergic response. Thus, itching and swelling remain the most direct indicators of an allergy in this context.