Thank you very much for your help!!
basically the tissue was fixed in 4% paraformaldehyde and embedded in paraffin. the sections are 8 micrometers thick.
the fluoroform of the secondary antibody that I used was alexafluor 488, the control was putting only the secondary antibody after the whole process of de-paraffin, rehydration, antigen retrieval and block, I saw fluorescence in the filter for GFP but also for Cy5, Rhodamine and DAPI.
In my sample with the primary antibody I saw higher fluorescence in GFP but the same fluorescence than my control in Cy5 Rhodamine and DAPI.
In addition I took another slide just after sectioning (that wasn't dewaxed) and the tissue showed the same unspecific fluorescence across the all wavelength.
should I fix it with another thing? do I need to add another chemical in order to reduce the that fluorescence? which could be?
Thank you very much again!!