Ricin comes from the castor bean plant, scientifically called Ricinus communis. The substance originates mainly in the bean but can be found in lesser amounts in other parts of the plant. Ricin is a toxin that causes death by interfering with the protein-making parts of cells.
Processed castor beans produce castor oil, a substance used around the world. Once the castor beans have been processed, a "waste" substance remains, which is where ricin is found. Intact castor beans do not release ricin; only when the beans are broken is ricin released.
When ricin enters the body, it goes to the cells and works on the ribosomes, the protein-making structures in cells. Proteins are not made, so cells die because all life processes are dependent on proteins.