It plays a role in virtually everything that goes on in your body. In your blood, it transports oxygen throughout your body. It builds and repairs tissue (including muscle tissue) and makes enzymes, hormones, and other body chemicals. It is also a fundamental building block of bones, cartilage, organs, skin, blood, hair, and nails.
The antibodies our bodies use to ward off disease and infection are made of protein, as are the enzymes that read the genetic information in our DNA to make new molecules. Messenger protein molecules transmit signals between cells, tissue, and organs, and transport protein molecules carry atoms and small molecules throughout the body.