On the surface the adaptive immune systems seems like a brilliant idea, but if you eat several raw chickens you don't become immune to salmoella, and you don't have to not wash your hands before eating for a few weeks to become immune to all the bacteria in soil etc. /as /i understand it this is because so many different strains of bacteria are involved, but my qustion is, if this is the case, then then adaptive immune system seems pointless, it can't really protect us from any more than a really tiny proportion of all the bugs that are out there, so why did it evolve in the first place?
The adaptive immune system doesn't function in such a way that exposure to one pathogen (disease causing organism like bacteria or viruses) protects you from all others. But exposure to one of these harmful organisms can trigger your immune system in such a way that you are a bit more protected against that organism next time you encounter it.
This is the principle that vaccines rely on - you are exposed to part of or the whole organism (that is usually treated in some way so it doesn't cause actual disease in you) and then next time you come across that organism your body is (should be) primed to deal with it and your immune system kicks in and starts protecting you before any signs of infection start to show.
The evolution of the immune system is a very complex chain of events and I don't pretend to be an expert but I can see why an adaptive immune system could confer some advantage to an organism.
Lets say that animal A gets infected by a type of bacteria. It survives the first infection. Animal A passed the bacteria around it's whole social group so they also became infected. Although animal A got over the disease, it has no adaptive immune system, meaning it is equally likely to catch the disease again. Which it does from another member of its social group. This time animal A is not so lucky and it dies.
If we repeat the scenario but with animal A this time having an adaptive immune system, animal A gets the disease the first time round but doesn't catch the disease again so doesn't die and can go on to have offspring, passing its genes onto the next generation.
This is a very simplistic explanation and I'm sure one of the other contributers can explain this process in more detail, but I hope it is of some help.
And using your example of chickens: I don't know if the same is true of salmonella but workers in poultry abattoirs often quickly develop a level of immunity to campylobacter (another type of food poisoning bacteria we catch from chickens)
Wikipedia entry on immunity: http://en.wikipedia.org/wiki/Immunity_(medical)
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