Re: How Does the white blood cells know which bacteria is bad to destroy them?

That's a very good question.

   Basically, the immune system is set up to distinguish things which are 
foreign to the body from the body itself.  In this way, if the body is 
invaded by bacteria, viruses, or fungus, the immune system will see the 
invader as foreign, and will know to attack it.  However, there are a couple of 
instances where this isn't totally true.  In certain parts of everyone's bodies 
there colonies of non-harmful bacteria, which are actually beneficial for the 
person.  One example is E. Coli bacteria in the intestine which aids the body 
in digestion and such.  It is not completely clear how the body knows not to 
always try to destroy this bacteria, but what seems to be the case is that 
the immune system can recognize certain pathogens as non-foreign, and so they 
don't get attacked.  This phenomenon is called "tolerance".

   One idea is that things within the body (foreign or not) have special 
chemical and biological "markers" and "tags", which the body will look for to 
distinguish which is which.  Its a complicated process to determine whether or 
not this is the case, so the search is on to recognize what these markers are 
and how can we use them.  Things like this would be valuable in curing diseases 
and infections.  Such as cancer or HIV where you might want to give some one a 
drug which will seek out the foreign invader and attack it specifically without 
attacking the body systems themselves.

   Another interesting fact about the immune system's ability to recognize 
foreign material in the body has to do with organ transplants from one 
person to another (for example, kidney or liver transplants).  Unless the 
transplanted organ comes from a genetically identical twin, the transplanted 
organ will be seen by the immune system as foreign.  It is still human tissue 
(as opposed to bacteria, etc) but every individual is genetically distinct, and 
so one person's immune system won't recognize someone else's organs as "self".  
With no intervention by doctors to control the immune response, organ 
transplants would never work because the recipient's immune system would always 
destroy the transplanted organ.  To get around this problem as much as 
possible, doctors do two things when performing transplants: 

     1)  Try to match the donor and recipient for certain genetic markers so
         that the rejection will be less intense, and 
     2)  Control the immune response of the recipient by giving them medicines 
         called "immunosuppressants".  These often work to prevent the  
         destruction of the transplanted organ.

Hope this answers your question.