How do allergies manifest on the skin?

Allergies often manifest on the skin through symptoms such as swelling and redness. This response is attributed to the body's immune system reacting to an allergen. The release of histamines in response to the allergen leads to inflammation and increased blood flow in the affected area, resulting in visible redness and swelling. This is commonly seen in conditions like allergic dermatitis or hives, where the skin becomes inflamed and puffy.

The other manifestations, such as dryness and flakiness, dark pigmentation, or scarring, are typically associated with different skin conditions or responses and do not directly correspond to the immediate effects of an allergic reaction on the skin. Therefore, swelling and redness serve as the primary indicators of an allergic reaction, making this the correct answer.