The tightly packed cells of the stratum corneum (top) provide a barrier against harmful material from the outside world, as well as protection against water loss.

It is also a highly effective barrier against the outside environment, being tough but flexible provided it is well hydrated. If its water content falls below 10% it becomes dry, less flexible and increasingly prone to damage, breakdown and infection.
    The epidermis as a whole is about 35 micrometres thick when dry, but can swell to 48 micrometres on full hydration. This depends more on the humidity and temperature of the surrounding air than on how much we have drunk!

The natural layer of oil-in-water emulsion on the skin is the first barrier against invasion by micro-organisms such as bacteria, fungi and yeasts. The stratum corneum provides the next level of defence.
    White blood cells in the skin can capture and destroy bacteria invading the epidermis. As a result pus may form.
    The epidermis also contains special defence cells (Langerhans cells) which are spread out amongst the keratinocytes. These cells mop up invading foreign substances that have found their way into the body, and take them off to special white cells (lymphocytes) in the lymph glands. Here they are neutralised.

A Langerhans cell in the skin.

    An important element of defence concerns chemicals. If a chemical such as nickel is constantly absorbed through the skin, say from a button on one's jeans, it is first taken up by the Langerhans cells; later, however, special lymphocytes called T-cells make antibodies to that chemical. This can in time lead to an allergic skin reaction at the site of the button as the T-cells rush to meet the invading chemical. Allergies are discussed further in Chapter 9 ('The safety of cosmetic products').

Functions of the dermis

These include:

• giving mechanical protection to the body from bumps and knocks; the collagen has an important role in this function