Specificity warning (Dobrucki): Specificity of labeling "should not be taken for granted." DRAQ5 binds DNA at low concentrations but also RNA when DNA sites are saturated. This is true of "most, if not all, fluorescent labels that exhibit affinity for DNA." For antibody-based IF, similar caveats apply: cross-reactivity with homologous proteins, non-specific binding in highly charged tissues, and background from Fc receptor binding on immune cells. Isotype controls and negative tissue controls are essential for validating specificity.
The technique selection framework (Combs & Shroff): For IF on thin tissue sections (4-5 µm), widefield fluorescence is often sufficient — the section is thin enough that out-of-focus blur is minimal. Confocal provides better optical sectioning for thicker sections or when axial discrimination matters. Two-photon excites deeper and bleaches less but is typically overkill for standard histology sections. The choice should match the sample: don't use a confocal for a 4 µm FFPE section when widefield gives equivalent results faster and more gently.
Signal amplification trade-offs: TSA (tyramide signal amplification) dramatically increases signal but introduces a trade-off: the amplified signal no longer has a linear relationship with target abundance. At high target densities, the amplification saturates — doubling the target doesn't double the signal. For binary classification (positive/negative), this doesn't matter. For quantitative measurement of expression levels, it limits interpretation. Standard indirect IF maintains better linearity but produces weaker signal.
Antibody specificity isn't guaranteed — cross-reactivity and non-specific binding can cause false positives. Controls are essential. For thin tissue sections (4-5 µm), widefield fluorescence is often sufficient — confocal is only needed for thicker samples or when depth discrimination matters. TSA signal amplification greatly boosts brightness but loses the linear relationship between signal and target abundance.
