The human body is well defended against infection. Sometimes pathogens like bacteria and viruses do get through, making you ill. There are physical barriers like your skin and the cilia (tiny moving hairs) and mucus that line your respiratory system. The mucus is sticky and traps bacteria and viruses that enter your nose as you breathe in. There are chemical barriers too such as enzymes in tears and saliva that break down pathogens. When the pathogens do get past the first line of your body's defenses, the second line is the white blood cells. There are several different types of these each... Show more

The human body is well defended against infection. Sometimes pathogens like bacteria and viruses do get through, making you ill. There are physical barriers like your skin and the cilia (tiny moving hairs) and mucus that line your respiratory system. The mucus is sticky and traps bacteria and viruses that enter your nose as you breathe in. There are chemical barriers too such as enzymes in tears and saliva that break down pathogens.

When the pathogens do get past the first line of your body's defenses, the second line is the white blood cells. There are several different types of these each working in a different way. Some of them engulf (surround) the pathogen and secrete enzymes that digest the bacteria, virus or single celled organism that has arrived in your body. Others neutralize any toxins that enter the bloodstream from the infection. Another type of white blood cell produces antibodies that react to a specific pathogen that enters your bloodstream. These antibodies then either damage the pathogen, making it harmless, or label it so that other cells can find and destroy it.

The white blood cells that produce the antibodies remain in your bloodstream so that the next time the pathogen gets into your body, it can immediately be tackled. They give you an immunity to disease. This natural defense can also be introduced artificially by vaccination. Some diseases are very harmful to the human body, for example, polio. This terrible disease left many children unable to walk properly until a vaccine was found. Nowadays children are vaccinated well before they can develop polio and it is virtually unknown in developed countries with a good health system.

But despite your natural defenses, infections do get through. If you eat a properly balanced diet and exercise regularly, your immune system will work well against invading microorganisms.

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