Which phenomenon shows up right away or several hours after UV exposure?

Immediate erythema refers to the reddening of the skin that occurs quite rapidly following exposure to ultraviolet (UV) radiation. This reaction is a direct result of increased blood flow to the skin as a part of the body's inflammatory response, aiming to repair any potential damage caused by UV exposure. The process can begin within minutes and typically peaks several hours after exposure, making it distinctive among skin reactions to UV light.

In contrast, delayed erythema manifests longer after the initial UV exposure—usually not appearing until 24 hours or more later. Skin sloughing refers to the shedding of dead skin cells and is generally associated with more severe damage or subsequent tanning processes, while immediate pigment darkening involves an immediate bronzing effect due to a temporary increase in melanin, which differs from the classic definition of erythema. Thus, immediate erythema is the phenomenon that correctly describes the immediate responses to UV exposure, distinguishing it from other skin reactions.