In the article, Paraffin Section Storage and Immunohistochemistry by Wester
et al, Applied Immunohistochemistry & Molecular Morphology 8(1): 61-70, 2000
they drew the following conclusions, "we conclude that storing of paraffin
sections: (1) results in, to a varying degree, a decreased immunoreactivity
for several, but not all, antigens, (2) influences both the extent and the
intensity of immunostaining outcome, (3) results in a considerable decrease
in immunoreactivity during the first 2 weeks of storage, (4) results in a
reduced immunoreactivity that is storage temperature dependent, (5) results
in a decreased immunoreactivity that can be compensated for by optimizing
the AR protocol, (6) results in a decreased immunoreactivity that is not
related to fixation time."

This is an interesting paper and is well worth reading. It confirms what has
been discussed so far, namely, that some antigens do diminish in stored
sections and that if you store the sections at -20 you can prevent this. Tom