The skin

The skin has many purposes, including:

  • acting as a barrier to protect the body from injury
  • keeping necessary fluids and proteins in the body
  • protecting the body from the harmful effects of ultraviolet (UV) light
  • helping control the body’s temperature.

The skin is divided into two main layers. The outer layer is known as the epidermis. The layer underneath is known as the dermis.

The epidermis contains three types of cell. On the surface are flat cells, known as squamous cells. Under these are rounder cells called basal cells. In between the basal cells are melanocytes.

The dermis contains nerve endings, blood vessels, and oil and sweat glands. It is held together by a protein called collagen.


Melanocytes are cells that produce a pigment called melanin. Melanin is responsible for the natural colour of our skin. It also protects skin from the harmful effects of the sun.

When our skin is exposed to sunlight, our melanocytes increase the amount of melanin. This is to absorb more potentially harmful ultraviolet (UV) rays. This makes the skin darker and gives it a suntanned appearance. A suntan is a sign that the skin is trying to protect itself.

If you have naturally dark (brown or black) skin, you have the same number of melanocytes as people with white skin. But your melanocytes make more melanin. This means you have more natural protection from UV rays.

Moles are a group or cluster of melanocytes that are close together. They are sometimes called naevi. Most people with white skin have about 10 to 50 moles on their skin. Some people can have as many as 100.

The structure of the skin
The structure of the skin

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Why do cancers come back?

Sometimes, tiny cancer cells are left behind after cancer treatment. These can divide to form a new tumour.