Wool has evolved to protect the sheep, keeping the animal cool in hot weather, warm when it is cold and dry in rain. These protective characteristics are why wool is such a valued fibre for clothing manufacturing.
Coarse wool tends to feel rough and hard against the skin. Fine wool is smooth and soft and doesn't 'itch' the skin. Merino sheep generally provide the softest wool of all sheep breeds.
Fibre Structure
Wool is mainly composed of something called Keratin, a long fibrous protein that is also found in the skin, hair, nails and horns of many animals - including humans.
Keratin is the substance that makes wool fibre waterproof, just as it does your skin!
The physical structure of a wool fibre is quite complex. It has an outer scaly layer and an inner core made up of two cell types, called the paracortex ('soft' keratin) and orthocortex ('hard' keratin). Because of their slightly different characteristics, these long cells wind together causing the wool fibre to develop a 'helical', or corkscrew-like form. This gives wool its incredible elasticity, although the helical form is not visible to the naked eye. What can be seen, however, are waves within the wool staple known as 'crimp'. This is a result of the wool's cellular structure, which helps the wool fibres to bind together when yarn is manufactured and gives the wool its soft, springy feel.