What is emitted by UVB radiation that affects skin pigmentation?

UVB radiation plays a significant role in the production of melanin in the skin. When the skin is exposed to UVB rays, it stimulates melanocytes, which are specialized cells located in the epidermis. These melanocytes then produce melanin, the pigment responsible for the color of skin, hair, and eyes. This process is a natural defense mechanism against UV radiation, as melanin absorbs UV rays and helps to protect deeper layers of the skin from damage, including potential DNA mutations that could lead to skin cancer.

Melanin production increases in response to increased UV exposure, leading to the tanning effect associated with sun exposure. This is why individuals often notice their skin darkening after spending time in the sun, as a result of the body's attempt to protect itself.

Other options like melatonin, keratin, and collagen are unrelated to pigmentation changes caused by UVB exposure. Melatonin is a hormone that regulates sleep-wake cycles, keratin is a fibrous protein that helps form hair and nails, and collagen provides structure to the skin but does not influence pigmentation. Therefore, melanin is the correct answer as it directly relates to the effects of UVB radiation on skin pigmentation.