How does our skin protect us?

Sunlight is such a double-edged sword, isn’t it? We love how its energy warms us and gives life to our planet, yet that same energy is incredibly damaging to the very skin that it warms. Our skin works remarkably hard to protect and defend against harmful UV rays, but the bad news is our skin’s defenses alone are not enough.

What happens when our skin is exposed to UV radiation?

Our skin defenses respond to UV light in two main ways: first, building up its shield so less damaging light gets into the skin and second, mobilizing a first-aid system to repair UV-damage from light rays that get past the shield.

You go outside. Sunlight beams down onto your exposed skin, and the harmful penetrating UV rays in that light are absorbed by skin cells. Sound the warning light - it’s time for damage control!

Our first response: melanin

Cue the first line of defense. The objective - block harmful light from penetrating the skin. Skin produces a pigment, called melanin, to absorb UV energy and divert it away from healthy cells. Skin cells first deploy melanin they have on hand and then go into melanin production to get more pigment to the skin.

This increase in melanin is responsible for the skin darkening that we call a suntan. It’s a fact that skin cancer rates are lower in people with darker skin. One hypothesis is that increased concentration of melanin helps to protect skin from damage (1). They literally have a sun-blocking “tan” built in!

Our second response: thickening

The second way our skin builds up its shield in response to sun is increasing thickness of the outermost layer of skin (called the stratum corneum), composed mainly of dead skin cells.

By increasing the thickness of this layer, harmful light has more layers of skin to pass through before causing damage to living cells. This is best thought of as your skin putting up a built-in sun umbrella.

Our third response: DNA repair

But alas, some UV light gets through despite our melanin production and thicker outer skin layer protection. UVB light causes direct damage to skin DNA, and UVA light generates “free radicals”, largely from oxygen, that damage DNA and proteins. It’s time to call in skin’s third and fourth responses - chemical repair teams to deal with the damage.

One repair team focuses on fixing the DNA damage while another team tries to rid the skin of those “radical” molecules. Let’s look at each briefly.

UV light causes changes in the normally smooth sequence of DNA, much like a force can put a kink in a smooth wire. Our skin has enzymes that can go in, cut out the kink, get replacement material that is the correct smooth fit, and weld the wire back together. But there are limits to what can be done and different people have different strength of enzyme response. If too much light is getting in, our enzymes can’t keep up.

Our fourth responses: antioxidants

While cells are attempting to repair DNA, they also release another repair team - the antioxidants. You’ve probably heard of antioxidants related to colorful foods like pomegranate and green tea. There are many types of antioxidants in our skin, all combating “oxidative” damage by free radicals.

Antioxidants mop up the free radicals by neutralizing them directly, or helping the cell perform chemistry to neutralize them (2). However, UV light is relentless. Over time, chronic sun exposure can reduce that cell’s antioxidant fighting capacity (3).

What if that's not enough?

If sun exposure is too high or protection too weak, despite our skin’s best efforts, sometimes damage is irreversible. If a cell is very badly damaged, the cell activates a program called apoptosis, or programmed cell death.

This is a controlled process by which the damaged cell is destroyed and recycled without causing damage to its healthy neighboring cells. It is very important to remove the damaged cells so they don’t start doing things they’re not supposed to, like growing without any controls, which can lead to cancer.

An incredible system that still needs support

In summary, millions of years of biological trial and error have given us a remarkable system in our skin that fights sun damage in multiple ways. But that system far from 100% effective.

Reflecting on how hard our skin works to protect us from damage should motivate us to help out by minimizing the amount of sun that gets to the skin and making sure the skin’s defenses are at full power to do their jobs!

What do you think? Have any questions about how our skin responds to UV radiation? Post in the comments below, and let's continue the discussion.

1. Agbai ON, et al. Skin cancer and photoprotection in people of color: a review and recommendation for physicians and the public. J Am Acad Dermatol. 2014;70(4) 748-62
2. Pisoschi AM, Pop A. The role of antioxidants in the chemistry of oxidative stress: A review. Eur J Med Chem. 2015;(97) 55-74 
3. Rinnerthaler M, et al. Oxidative stress in aging human skin. Biomolecules. 2015; 5(2)545-89