The primary type of structural protein found in hair and nails is keratin. Keratin is a fibrous protein that provides strength, durability, and protection to these structures. It is a key component of the outer protective layer in hair, nails, and skin, contributing to their toughness and resilience against physical and environmental damage. Keratin's unique structure includes a high degree of disulfide bonds, which contribute to its stability and ensure that hair and nails retain their shape and integrity over time.