Imagine your body is a fort under attack from viruses and bacteria. You have two lines of defense. First, your skin and the mucous membranes in your respiratory tract literally screen out germs. If these lines of defense are penetrated, the immune system kicks in, sending white blood cells to the affected region. Backup troops -- specialized white blood cells, proteins in the blood known as antibodies and other blood components -- follow, working in an orchestrated effort to strike down the invaders.

1 The attackers

Viruses and bacteria need an entry portal, such as irritation or dryness in the mucous membranes or an opening in the skin. Normally, mucous membranes contain immunoglobulin A, which kills off invaders. If there is an entry portal, or a particularly large load of virus or bacteria (someone with a cold sneezes in your face), invaders can break through the barriers.

How fever begins:
White blood cells release endogenous pyrogens that work on the hypothalamus to raise your temperature. Most viruses and bacteria can't thrive in hot environments. Some doctors suggest NOT taking fever reducers unless your temp is very high -- say, over 101 degrees.

2 The defenders
Antibodies in the blood recognize the invaders as foreign. A chain reaction then occurs that causes white blood cells stored in the blood vessels, spleen and bone marrow to rush to the point of entry.

3 THE "KILL"

  At the entry point, white blood cells literally swallow the invaders, releasing powerful substances to destroy them.

ŠUSA Weekend Jan. 7-9, 2000