The chemical building blocks from which mammalian cells, organs, and tissues like muscle are made. Proteins also serve double-duty as hormones, enzymes and antibodies, which help fight off invading germs. Proteins are made of long chains of even smaller building blocks called amino acids. Amino acids determine the size, shape, and length of protein molecules. They also give protein molecules the odd ability to coil and uncoil like tiny, cellular snakes.