Antibodies are proteins that are produced by the immune system and help the body to fight foreign substances, such as bacteria or viruses. HuM195 was made by putting human leukemia cells into mice. Most of the mouse parts of this antibody were replaced with human parts. Only the part of the antibody that binds to the leukemia cells was kept from the mouse. HuM195 attaches to leukemia cells but does not attach to most normal cells. It can kill small amounts of disease by identifying the leukemia cells as "foreign." HuM195 has worked less well against large amounts of leukemia since the normal immune cells needed to kill leukemia cells are lowered in most patients with leukemia.