MECHANISMS TO ATTACK INVADERS

Vertebrates in general, and mammals in particular, have amazingly effective immunological defenses that are in essence a system of carefully targeted chemical warfare. Cells called macrophages constantly wander the body searching for any foreign protein, whether from a bacterium, a bit of dirt in the skin, or a cancer cell. When they find such an intruder, the macrophages transfer it to a helper T cell, which then finds and stim- ulates whichever white blood cells can make a protein (called an anti- body) that binds specifically to that particular foreign protein (an antigen). Antibodies bind to antigens on the surfaces of bacteria,

thereby impairing the bacteria and also labeling them for attack by specialized larger cells. If the antigens persist, say during a continuing bacterial infection, they stimulate the production of ever more of the cells that make that specific antibody, so that the bacteria are destroyed at an ever-increasing rate. Whatever is recognized as a properly functioning part of the body is permitted to remain. All else-disease organisms, cancerous tissue, organs transplanted from other individuals-is attacked.

How does the body recognize cells as its own? Each cell has a mo- lecular pattern on its surface, called the major histocompatibility com- plex (MHC), which is like a photo ID card. Cells that have a valid MHC are left alone, but those that have a foreign or missing MHC are attacked. Interestingly, when cells are infected, they transport protein from the invader to the MHC, where it is bound. Like indi- viduals with obviously fake ID cards, such cells are priority targets for the killer cells of the immune system. The adenovirus, a common cause of sore throats, has found a way to get around this defense. It makes a protein that blocks the ability of the cell to move foreign proteins to the MHC. In essence, it prevents the infected cell from signaling that it has been invaded.