Natural barriers

The body has natural barriers to stop harmful microbes getting inside the body. Here are some of them:

• acid in the stomach kills many microbes

• sticky mucus in the lungs traps microbes, and then cilia sweep it out of the lungs

• the skin stops microbes from getting into the body

• scabs form on the skin if you get a cut, stopping microbes from getting into your body

• tears contain substances that kill bacteria

The immune system

The body has an immune system that kills microbes if they get past the natural barriers. White blood cells are very important in the immune system. There are different sorts of these cells, but they can do two main jobs.

• Some white blood cells can engulf microbes and kill them.

• Some white blood cells can make substances called antibodies that stick to microbes.

Microbes have chemicals called antigens. Different microbes have different antigens. White blood cells have chemicals called antibodies.

White blood cells can stick to microbes if they have the right antibody to match the antigen on the microbe. When this happens the microbes can be killed, or clumped together to make it easier for other white blood cells to kill them.