Cat hair, coloured scanning electron micrograph (SEM). The outside of the hair, the cuticle, is covered in overlapping scales of dead cells containing the protein keratin. Found exclusively in mammals, hair is one of their defining characteristics. This hair is from the British Blue (British Shorthair) breed of the domestic cat (Felis silvestris catus). The hair in cats serves as insulation and can be raised when threatened to give an appearance of increased size. Cat hair is a common cause of allergies in humans. Magnification: x265 when printed 10 centimetres wide.