Antibodies are highly specific proteins in the immune systems of complex organisms. They recognize and combine with bacteria, viruses, and other foreign proteins to defend against intruders and protect against disease. Proteins are crucial components of muscle. Muscle contraction results from the sliding of two different protein filaments past each other. The movement of chromosomes during mitosis and the action of sperm flagella as the swim are produced by contractile structures consisting of proteins.
Proteins are important in the proper and efficient functioning of sight, hearing, and other senses.
Many proteins are simply structural substances, providing the filamentous structure within cells and the materials used in hair, nails, tendons and bones of animals. The high strength of skin and bone is due to the presence of collagen, a fibrous protein. In nature, structure and function are closely related. The structure of an organism, organ, tissue, cell, organelle, or molecule has a great influence on how it works and what it can do. In spite of the diverse nature of protein function, all proteins have the same type of linear polymer structure built with various combinations of the same 20 amino acids.