1. A computer-implemented method of processing image data representing biological units in a tissue sample, the computer including a processor, the method comprising: receiving, by the processor, a first image of the tissue sample containing signals from an immunofluorescent (IF) morphological marker, wherein the tissue sample is stained with the IF morphological marker; receiving, by the processor, a second image of the same tissue sample containing signals from a fluorescent probe, wherein the tissue sample is hybridized in situ with the fluorescent probe; classifying, by the processor, each biological unit in the tissue sample into one of at least two classes based on a mean intensity of the signals from the IF morphological marker in the first image; performing, by the processor, a fluorescence in situ hybridization (FISH) analysis of the tissue sample in the second image to obtain results therefrom; and filtering, by the processor, the results of the FISH analysis to produce a subset of the results pertaining to biological units classified in one of the at least two classes only.