Germs are everywhere: on your hands, in the air, in food. In Helpful and Harmful Microbes you met the ones that can make us sick. Here is how the body keeps most of them out and fights the ones that get in. The fighters travel in the blood, from The Heart and Blood.
Key idea
Barriers come first: skin, mucus and tiny hairs in the airways, tears and stomach acid keep most germs out. White blood cells attack germs that get in. Some white blood cells remember a germ, so the body responds faster and harder the next time it meets the same one.
Why it works
The body has two layers of defense. The first responds the same way to any invader: barriers, then white blood cells that swallow germs, plus inflammation (swelling, redness) and sometimes fever. The second layer learns: certain white blood cells make antibodies that fit one particular germ, and memory cells keep that pattern for years. Vaccines work this way: they show the body a harmless look-alike of a germ (a weakened or killed germ, or just a piece of one) so memory forms without the illness.
Worked example: Defense or germ?
You have a cold. Your nose runs and you have a mild fever. Are those the virus itself?
- A cold virus infects cells lining the nose and throat.
- Extra mucus traps germs and washes them out; a higher temperature is part of the body's defense.
Answer: No. They are signs of the body's defenses at work.
Worked example: Why faster the second time?
Why do people usually get chickenpox only once?
- During the first illness, the body makes antibodies against the chickenpox virus.
- Memory cells stay in the body.
- If the virus comes back, the memory cells respond before it can spread far.
Answer: Memory cells from the first time respond quickly.
Common mistake
Students think a fever or a runny nose is the germ itself, or that the body has only one kind of defense. Symptoms are often the body fighting back. And there are layers: barriers, the general attack, and the learned, remembering response.
Washing hands with soap and water removes many germs before they can get in (CDC). Antibiotics act on bacteria, not on viruses such as colds and flu.