Which type of UV light causes the skin to close up around keratinocytes?

The correct answer is UVA. This type of ultraviolet light penetrates deeply into the skin, reaching the dermis and affecting the living cells, including keratinocytes. When skin is exposed to UVA radiation, it can lead to a series of biological responses that can result in the skin tightening around keratinocytes. This is part of the skin's protective mechanism and can contribute to skin aging and damage over time if exposure is excessive.

On the other hand, UVB primarily affects the outermost layer of the skin and is responsible for causing sunburn and initiating the production of vitamin D. UVC is mostly absorbed by the Earth's atmosphere and doesn't reach the skin under normal circumstances, making it less relevant in this context. Infrared light, while it can cause heating effects in the skin, does not interact with keratinocytes in the same manner as UV light, particularly UVA, which specifically influences skin structure and function. This understanding highlights the unique role of UVA in skin biology and its implications for tanning and skin health.