Cancer plays a deadly game of hide-and-seek in the body, and the drugs sent to treat it are often the losers—as is the cancer patient. The drugs have trouble distinguishing between tumor cells and healthy ones and may drop their payload on the normal cells, causing miserable side effects and leaving nearby cancer cells untouched. Malignancies may also get a helping hand from the body's own leading defense weapon, the immune system. It often mistakes anticancer drugs for harmful bacteria or other foreign invaders and breaks them down. The shattered pieces are conveyed to the body's trash receptacles in the liver, kidneys and spleen, again, before they reach their intended target. Even when the drugs do manage to arrive at a tumor, many of them become entangled in the dense undergrowth of the malignant mass—unable to penetrate it completely.