The way it was explained to me, is that it is possible for each individual 
block to show a false positive, therefore, a negative is ran with that 
species type antibody on each block used to verify that if there is positive 
staining, that it in fact is a true positive staining.  Also, it was 
explained to me that if you are running a variety of stains, and some are 
mono-clonal (mouse) and some poly-clonal (rabbit), you will need to run a 
negative mouse and rabbit on that one block.  The reason being, if you ran 
all of the stains and just one negative, say a mouse, the mouse negative may 
show that there is no false positive staining when in fact if a negative 
rabbit had been ran, there very well could have been false positive staining 
in a certain area, therefore the poly-clonal antibody may have shown to be 
positive, when in fact, it was a false positive that went un-detected because 
there was not a negative rabbit ran with that case.  That is the reason for 
running the negative slides on each block.  I hope that I did not go on too 
much.  

Jeanie Wade, H. T. (ASCP)