5.2a Homeostasis in an organism is constantly threatened. Failure to respond effectively can result in disease or death.

5.2b Viruses, bacteria, fungi, and other parasites may infect plants and animals and

interfere with normal life functions.

5.2c The immune system protects against antigens associated with pathogenic organ-

isms or foreign substances and some cancer cells.

5.2d Some white blood cells engulf invaders. Others produce antibodies that attack

them or mark them for killing. Some specialized white blood cells will remain, able to

fight off subsequent invaders of the same kind.

5.2e Vaccinations use weakened microbes (or parts of them) to stimulate the immune

system to react. This reaction prepares the body to fight subsequent invasions by the

same microbes.

5.2f Some viral diseases, such as AIDS, damage the immune system, leaving the body

unable to deal with multiple infectious agents and cancerous cells.

5.2g Some allergic reactions are caused by the body’s immune responses to usually

harmless environmental substances. Sometimes the immune system may attack some of

the body’s own cells or transplanted organs.

5.2h Disease may also be caused by inheritance, toxic substances, poor nutrition, organ

malfunction, and some personal behavior. Some effects show up right away; others may

not show up for many years.

5.2i Gene mutations in a cell can result in uncontrolled cell division, called cancer.

Exposure of cells to certain chemicals and radiation increases mutations and thus

increases the chance of cancer.

5.2j Biological research generates knowledge used to design ways of diagnosing, pre-

venting, treating, controlling, or curing diseases of plants and animals.