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 stimulates
whichever white blood cells can make a protein (called an antibody)
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 molecular
pattern on its surface, called the major histocompatibility complex
(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 individuals
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.