How does the immune system monitor cells to determine if they are “self” or “non-self”?
A. Within all cells there is an ingenious mechanism that takes proteins from inside and displays them as a kind of `flag,’ announcing that cell’s allegiance to self. This flag is called the histocompatibility antigen. When the immune system finds cells displaying a foreign flag (either because they are infected or have become malignant) in addition to the flag of self, it sends lethal T cells to kill them on contact. Q. So how does cancer evade this sophisticated surveillance system? A. Normally, it doesn’t. But evolutionary pressures are at work. Some malignantly transformed cells acquire the means to evade destruction. They release a barrage of non-self material, waving foreign flags, into the blood. This tricks the immune system into switching production from effective cellular immunity to useless humoral antibodies. As we said, humoral antibodies cannot kill cancer cells. So thanks to this ruse, and there are probably others, malignant cells can multiply and form tumors. Q. How can
