cancer involves our cells losing their ability to control their own growth. And the body has a hard time distinguishing cancerous cells from normal cells, so that drug design for cancer is difficult.
Cancer cells, even within the same tumor, are heterogeneous—that is, differences exist between the individual cells. The consequences of this fact started coming into focus only a few years ago, when researchers showed that cells collected from four different regions of the same tumor were in fact quite different. Further studies have reinforced this finding, and cancer cell heterogeneity is now widely recognized. Given that biopsies are typically taken from a single spot within a tumor, this fact has serious implications for improving diagnostics and therapies. It also indicates that any one targeted therapy is highly unlikely to eliminate all cancer cells by itself.
Cancer cells, although different in many ways from other cells in the body, are known to evade our immune system or suppress key elements of the usual immune response. In some cases aggressive cytotoxic (killer) T cells—the immune cells that locate and kill invading pathogens—actually infiltrate tumors. For some reason, however, they soon halt their attack through a combination of cell-to-cell signaling and an influx of T regulator cells, a different type of immune cells that suppress the immune response. Other research found that a chemical compound is sometimes added to cancer cell DNA and suppresses the activity of certain genes, making the cells much less likely to be targeted by the immune system.