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How we fight bacteria: our immune system vs bacteria

Bacterial infections have always existed, and all organisms that can become infected have learned to deal with them. Plants have primitive defenses against bacteria by producing toxic substances. Higher animals and humans fight a bacterial infection with a more specific immune system.
Our defenses against bacterial infection are active on several levels.

Level 1: Prevention and removal.

Our first defense is to prevent the survival and entry of pathogenic bacteria on and inside our body. Since the outside of our body is most likely to get in contact with bacteria, that is where this defense is operative. Our skin offers perfect protection: no bacteria can penetrate intact skin. Sweat increases the salt concentration to levels that most bacteria find rather hostile. Though you may have never looked at it that way, our mouth and lungs, and even our intestines could be regarded as parts of our 'out'side since they are in contact with the environment. Such exposed areas are covered by mucus. All our 'openings' are protected by a constant removal of bacteria: our eyes produce tears, which contain anti-bacterial substances, and saliva removes bacteria from our mouth when we swallow. Minute hairs on the inside linings of our ears and airways remove, by their constant movement, dust particles and bacteria that are trapped in mucus. Even our urinary tract is flushed each time we go to the toilet. Finally, the acidity of our stomach kills most germs we ingest. The presence of 'healthy' bacteria in our intestines protects us to a large extend against pathogenic bacteria. All these defenses are passive and not specific, but very effective.

Level 2: Non-specific killing of bacteria.

As if level 1 defense is not enough (and sometimes it isn't), our body has developed a non-specific, ever active, lethal weapon for those bacteria (and viruses as well), that succeed in entering sites where they can cause harm. This weapon is comprised of a number of compounds in our blood and bodily fluid called complement, that kill most bacteria when it comes into action. Moreover, certain white blood cells inspect our body constantly to scavenge for unwelcome guests, which are eaten alive when discovered. These defenses comprise our so-called innate immune system. Similar defense strategies are found in lower animals, and they can be seen as early evolutionary solutions to deal with infectious diseases.
Level 3: acquired immunity
Some bacteria, mean creatures as they are, can break through our defense levels 1 and 2, and grab their chance to make us ill when they can. Our body raises the temperature to slow down bacterial growth (that is why a fever can help you fight an infection, as long as it doesn't get out of control). But ill as we are, our body brings into action the reserve troops of our defense system. It needs time to recruit the best we have, but after 2 weeks specific antibodies are produced, which recognize exactly those bacteria that cause the infection, and with the help of white blood cells they are destroyed. Read more about cell-mediated immunology ((source: J Kimball). It is this level 3 defense system, called acquired immunity, which allows many bacteria to make us ill only once in life. The bacteria do not get a second chance because then the response time is reduced to nearly zero: our body remembers the infections it has seen, and the bacteria are killed before they can do harm.
It is this ability that is used to produce immunity by vaccination: before the bacteria make us ill, we activate our immunesystem to recognize and remember them, so that when struck, our body can strike back immediately. Since antibodies are highly specific, we need a different vaccine for each bacterial infection that we want to become immune to.
The immune system is a very complicated biological interplay. Much of our understanding and knowledge comes from the study of cases where something is not working properly. We can understand and appreciate the immune system by knowing the effects of malfunction. Various ailments can be the result, for instance those listed in this brief summary on immunological disorders (Source: NCBI).  Immunological diseases can result in a decreased immunity which makes us infection prone. In the most severe form SCID (severe combined immunodeficiency) patients suffer from life-threatening infections. The SCID Home page contains many links to sites with more information on this subject (Source: B Ballard). In the reverse situation, immunological disorders can lead to over-reactivity, for instance to proteins in our own body (auto-immunity). Lupus is an auto-immune disease, and at this Lupus Home site the role of the immune system is explained (Source: Uni Birmingham). You can find a list of immune system diseases here (Source: Hardin MD). Immuneweb is a site where patients suffering from immunological disorders can meet (Source: immuneweb.org).
The immune system can be helped in the battle against bacterial infections with antibiotics. Read more about how we can help our fight against bacteria in our other exhibits.


The content of this page is copyright of the Virtual Museum of Bacteria and was written by Dr. T. M. Wassenaar (curator), with support of the Foundation for Bacteriology.

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Page last modified: 09 Mar 2007

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