The word 'protein' comes from a Greek word that means 'of prime importance'. Proteins help us see, contract our muscles, and clot blood. Our organs and immune system are made up of mostly protein. Learn what a protein is and the foods that provide it, how our bodies process protein, the difference between high and low quality proteins, and the recommended intakes of this essential nutrient. Learn what the best dietary sources of protein are and how our bodies use amino acids.