The immune system protects organisms from infection with layered defenses of increasing specificity. Most simply, physical barriers prevent pathogens such as bacteria and viruses from entering the organism. If a pathogen breaches these barriers, the innate immune system provides an immediate, but non-specific response. Innate immune systems are found in all plants and animals. If pathogens successfully evade the innate response, vertebrates possess a third layer of protection, the adaptive immune system, which is activated by the innate response. Here the immune system adapts its response during an infection to improve its recognition of the pathogen. This improved response is then retained after the pathogen has been eliminated, in the form of an immunological memory, and allows the adaptive immune system to mount faster and stronger attacks each time this pathogen is encountered.
In short, the innate immune system responds the same way to each new attack and wakes the adaptive immune system to action. The adaptive immune system changes in response to new threats and remembers how to respond to old threats. This process of adaptation is in fact what brings that component of the immune system to its knees eventually. But there are different and less well understood mechanisms at work that undermine the innate immune system:
Courtesy of http://www.fightaging.org/
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