Did you know your skin has three layers? Hassan’s genetic mutation kept the outer layer of his skin from attaching to the inner layers. That made his skin very fragile. Each layer of human skin has a lot of jobs to do—and they must work together to keep a person healthy.

Epidermis. This is the only part of skin you see. New skin cells called keratinocytes—the same type found in hair and fingernails—form at the bottom of the epidermis. About a month after they form, they have traveled to the top of the skin. Then they flake off. They join the billion tons of dead skin cells floating in the atmosphere worldwide. (Did it take you a minute to read this paragraph? During that time, you lost around 50,000 skin cells from your epidermis.)

The epidermis also has Langerhans cells. Those send your immune system warnings: “Watch out! A virus is here!”

Dermis. A lot of work happens one level deeper, in the dermis. Little glands here make oil to keep your skin soft. Sweat glands in the dermis do double duty—cooling you off with perspiration and getting rid of waste fluids. Nerves in the dermis send messages to your brain about how things feel against your skin. (Itchy? Hot? Pleasant?) You’ll also find tiny sacs in the dermis called hair follicles. Hairs start here, then travel up through the epidermis.

Subcutis. This bottom skin layer is mostly fat. It keeps you from getting too hot or cold. The subcutis connects your dermis to your muscles and bones and helps protect them from bumps and bruises. This fatty layer is even designed to save your life. If your body runs out of food, it will burn up some of this extra fat for energy.