Keratin is the protein that gives human hair and nails much of their strength.
Keratin is a tough, fibrous structural protein produced by specialized cells called keratinocytes. In hair and nails, densely packed keratinized cells form durable structures that protect underlying tissue. Hair is mainly made of hard keratin, while nails contain a particularly compact form of the same protein.
Keratin also occurs in the outer layer of skin and contributes to the protective barrier against friction, water loss and some environmental damage. Its strength comes partly from bonds between amino-acid chains, including disulfide bonds formed by the amino acid cysteine. More disulfide bonding generally makes keratin structures harder and less flexible.
Collagen is a plausible alternative because it is another abundant structural protein, but it is especially important in connective tissues such as tendons, skin and bone. Elastin provides stretch and recoil, while hemoglobin carries oxygen in red blood cells; neither is the main strengthening protein in hair and nails.