The disease-fighting immune system is programmed to destroy abnormal cells, like cancer cells. Scientists now know normal cells begin to split and produce copies of themselves through a complicated process. Cells produce substances called growth factors. These substances bind to receptors on the surface of cells, like keys fitting into locks. Growth factors trigger cell division.
Cancer cells flaunt the rules.
Hundreds of thousands of cancer cells combine to form a tumor. Once the tumor grows big enough, bits of it can break off and spread through the bloodstream to other body parts. There the bits seed, or sprout, new tumors. Scientists have also begun to zero in on how genes, the cell parts that contain all inherited information, impact human cell division. They know hundreds of genes play different roles in the process.