Cat hairs. Coloured Scanning Electron Micrograph (SEM) of shafts of cat hairs. Cat hair is a common cause of human allergies. Here hairs of different thicknesses are seen. Each hair shaft has an outer cuticle made up of flat overlapping scales or plates. The plates are thought to prevent the hairs from matting together. Hair, formed from the protein keratin, is made up internally of three concentric layers called the medulla, cortex and the outer cuticle. These tiny hairs may provoke an allergy to the eyes, nose or respiratory tract of people that have a hypersensitive reaction to cats. Magnification: x250 at 6x7cm size.