To explain what happens in eczema, you need to understand the structure of the skin as seen down a microscope. The skin consists of three layers:

• The outer layer is called the epidermis. This contains a ‘brick wall’ of skin cells (keratinocytes) that are held together by a cement (the mortar) mostly made up of fats or lipids. The many different layers start with live cells that constantly reproduce, creating new cells that move up to the surface, die and are shed.

This whole process takes about 28 days. The lipid cement makes the brick wall into a very effective barrier against the environment. It prevents the skin losing too much water and prevents noxious (poisonous) substances getting in.

• The middle layer is called the dermis. This consists of tough structural fibres called collagen and elastin, which provide strength and elasticity to the skin. It also contains blood vessels that supply nutrients and oxygen to both the dermis and the epidermis.

• The deepest layer of the skin is called the subcutis and is predominantly made up of an insulating layer of fat.

In eczema, it is the dermis and epidermis that are affected. The epidermis shows the most marked changes. The inflammation leads to leaky blood vessels, so fluid collects between the keratinocytes, causing them to separate. The brick wall takes on a sponge-like appearance.

As the eczema becomes chronic, the constant rubbing and scratching causes the epidermis to regenerate more quickly, so it becomes thickened. Finally, eczema causes changes in the upper part of the dermis. This region becomes flooded with white blood cells, which are part of the body’s immune system or defences.

They leak out of vessels and even pass up into the epidermis. Current evidence suggests that it is these cells that drive the whole process of inflammation in the skin.

Is the skin just a simple barrier?

No, it is much more than that, and this accounts for the many physical and psychological effects that it can have, especially if you are badly affected by eczema. It is the last line of defence against the outside world, protecting our bodies from external attack and keeping the right conditions inside (homeostasis) through its prevention of loss of fluid and regulation of temperature.

It also allows for a display of individuality through decoration, jewellery and hair-styling. Skin, therefore:

• acts as a barrier to physical agents including ultraviolet radiation
• protects against mechanical injury
• defends against microbes
• is involved in homeostasis – preventing the loss of water and electrolytes
• regulates temperature and insulates
• is involved in sensory functions
• is integral to fine touch and grip
• is the site of vitamin D synthesis
• acts as a calorie store in the subcutaneous fat
• has cosmetic, psychosocial and display functions.

Why does my skin weep fluid and feel wet?

If you think of the epidermis changing to look like a sponge, you can imagine the fluid leaking out and making the outer layer stretch up into blisters. Once these break, you will be left with a wet weeping area as the skin has lost its barrier function.

How does our doctor know that my child’s rash is eczema?

As there are no specific tests for most types of eczema, your doctor will have reached a diagnosis on what we call ‘clinical grounds’. This means taking a careful history of the problem and any family history of eczema, asthma or hay fever. Examination of the skin will add to the clues in the history, allowing a diagnosis to be made.

Some people with atopic eczema may have abnormal blood tests, such as high levels of an antibody called immunoglobulin E (IgE). Antibodies are chemicals made by the body as a defence against infection but are also involved in allergic reactions.

Specific allergens (the causes or triggers of allergy) can lead to high levels of linked IgE, which can be measured by a blood test called an ELISA test. These tests do not, however, diagnose atopic eczema as you can have abnormal tests and never develop eczema, or have normal IgE levels and very definite eczema.

My family are from India and I have noticed that my eczema looks different from eczema in my friends with white skin. Why is this?

I am afraid that we cannot explain why there are different patterns of eczema in different racial groups, but you are right to have spotted the difference. Eczema often affects the flexures (the creases in front of the elbows and behind the knees), but in Asian and particularly in African/Caribbean people, eczema sometimes shows a reverse pattern, affecting the extensor surfaces (behind the elbows and the front of the knees).

There can be other differences in the way in which pigmented skin reacts giving rise to unusual presentations of eczema:

• Thickening of the skin (lichenification) seems to happen much more readily.
• Lumps or papules are more common, giving a raised, bumpy appearance to the skin.
• There may be a marked increase or decrease in pigmentation of the skin after the eczema has settled down. This can be very distressing as it can be quite disfiguring.

Why does skin itch with eczema?

Surprisingly, this is a very difficult question to answer as the current scientific understanding of itch is really very poor. We do know that certain small nerve fibres in the skin transmit ‘itch’ signals to the spinal cord and then to the brain. These same fibres can also transmit pain signals.

There are certain centres in the brain that receive these signals and then interpret them either as an itch sensation or sometimes as pain. Two different types of nerve fibre are involved, one being faster than the other. This explains why itching can be made up of an early localised pricking sensation followed by a diffuse itching or burning sensation.

Why people with eczema itch isn’t really known, but it may be that the dry, inflamed skin of eczema fires off these nerve fibres, causing the itch. There is, however, also some evidence that these nerve fibres and the chemical signals (neurotransmitters) that they contain may be abnormal in eczema.

This could mean that the abnormal itching sensation is the first problem, with the other skin changes being ‘secondary’ – i.e. being the result of the damage caused by scratching. A lot more research needs to be done into the mechanisms of itch before we have a clearer picture.