I want to ask if antigen retrieval process works for formalin fixed tissues of more than 7 days or even 2-3 months ? Or it should be done for tissues that are fixed for max upto 24 hours ?
Yes. I've done this with samples collected on ships, stored in formalin over long periods during the collection voyages, and then brought into the lab for embedding and IHC/IF. Microwave antigen retrieval with citrate buffer works well for the antigens/antibodies we were interested in. Like all IHC/IF procedures, our extended fixation protocol may not work for the antigen/antibodies you are interested in.