Hi Michelle,
Your number one concern for your controls is to have them fixed and
processed as close to your test tissue as possible. So keeping the
control material in formalin for any longer than a few days is not
really acceptable. The longer the tissue is kept in formalin, the more
permanent the crosslinkages become and harder to unmask the antigenic
sites. Although heat induced epitope retrieval has made that easier in
many instances. I think it is best to fix optimally (12-24 hours, if
most of your specimens are fixed approximately that long) and process
your controls to paraffin and store blocks in a dry, cool place.
This applies to special stains also, not just immunos. Quality of many
stain reactions suffer from overfixation in formalin.

Regarding immunohistochemistry, I have tonsil control blocks for
instance, fixed in formalin at different intervals from 6 hours up to 78
hours to match up with test tissues fixed only a few hours and up to a
long weekend (3 days) to do light and heavy chains on lymphoma cases.

I hope this helps, Katri.

Katri Tuomala
Anatomic Pathology
St.Joseph's Hospital
Hamilton, Ontario, Canada