Wool fibers have microscopic scales that when agitated and then exposed to water and heat tend to stick together causing the shrinkage. The edges of the scales in wool fiber interlock during the washing process, which prevents the fabric from returning to its original size once it has dried. This effect is also known as felting.
Wool originally refers to the fleece of sheep. In the textile industry, however, wool fabric has also come to include the hair fibers taken from other animals such as alpaca, vicuna, angora goat and camel.
Hair that is harvested from Angora rabbits is not classified as wool, although the look, appearance and feel of the resulting fabric bears strong resemblance to wool. The fabric produced from the Angora rabbits also tends to shrink the most compared to any other type of wool fabric.