Hi,  I think that some sort of protein block in diluents and detergent in the solution are basics, as well as some sort of block for the endogenous enzyme that is your localisation label.
After that, then the pretreatment may lead to background, if microwaving etc is too long, or we have been diluting commecial target retrieval solutions to abolish background, and reducing the power level of microwaving.   It is variable, some tissues give no bother, whilst others....
And then we can add endogenous biotin as another cause, particular antibody dilutions 
The variability you have with cell blocks will be a function of the different material, fixation and processing.

Dave Edmondson
Christie Hospital, 
Manchester. UK
-----Original Message-----
From : “Scholz, Stephen J.“ 
To : Histonet@Pathology.swmed.edu
Date : 05 December 2003 17:32:06
Subject : [Histonet] pretreatment for IHC
Hello Histonet,
>
>I am having a problem with background staining on IHC slides.  This is only happening in cell block specimens.  Is this common on cell blocks?  Should I do a pretreatment on these?  What pretreatments do others prefer?  Should I peroxide quench?  Before or after?  What about blockers?  Do I ramble on and ask to many questions?
>
>Thanks for all your help,
>
>Stephen J. Scholz HT(ASCP)
>Histology Coordinator
>OSF St. Anthony Medical Center
>Rockford IL
>
>Phone: 815-395-5410
>Fax: 815-395-5364
>e-mail: sjscholz@osfhealthcare.org
>
>
>


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