A protein found in skin, nails, hair and teeth, keratin is responsible for the skin's structural integrity. It also forms a protective barrier on the outer layer of the skin, according to Teens Health. Keratin can be hard, as in the case of fingernails, or soft like skin, says Wikipedia.
Made up of tough, insoluble filaments, keratin is extremely difficult to dissolve and requires a chemical reaction to do so, according to Wikipedia. Keratin is produced by cells called keratinocytes and contains amino acids, a specialized type of protein, according to Teens Health. Keratin can also be found in animal hooves and horns.