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On a tissue section, if you want to know where a particular protein sits and how much of it there is, routine staining will not help — it only shows the general outline of cells. Pinpointing one specific protein relies on antibodies: their specific binding to antigens turns an invisible molecule into a visible signal. This is immunohistochemistry and immunofluorescence.

1. The principle: antibodies lead the way, labels emit the light

The idea can be broken into two steps:

• Recognition: a primary antibody binds specifically to the target protein (antigen), a specificity determined by the antibody itself;

• Visualisation: a labelled secondary antibody then binds the primary antibody, and the label converts the binding event into a visible signal.

Depending on the label, there are two main routes:

• Immunohistochemistry (IHC): the label is an enzyme (commonly horseradish peroxidase, HRP); adding a chromogenic substrate produces a coloured precipitate that can be viewed on an ordinary brightfield microscope;

• Immunofluorescence (IF): the label is a fluorophore that emits light under excitation of a specific wavelength and requires a fluorescence microscope.

2. What IHC and IF are each good for

• Signal and viewing: IHC gives a coloured precipitate under brightfield, and slides can be archived long term for pathology; IF gives fluorescence on a dark background with high contrast.

• Multiplexing: IHC makes it difficult to label several proteins at once; IF makes multicolour easier.

• Quantification: IF fluorescence intensity is convenient for quantitative analysis, though it is affected by exposure and photobleaching; IHC staining intensity can also be scored, but is more subjective.

• Equipment: IHC needs an ordinary brightfield microscope; IF needs a fluorescence microscope whose filters match the fluorophores in use.

3. Multiplex immunofluorescence: seeing how cells sit next to each other

In cancer research and elsewhere, what matters is often not a single protein but the relationships among several cell types in tissue. Multiplex immunofluorescence (mIHC) labels several targets on one section, shows each in a different fluorophore, and combines the channels so you can see which cell types sit next to which, and whether immune cells have entered the tumour region.

4. Which steps matter most

For formalin-fixed, paraffin-embedded tissue, a typical workflow runs: sectioning → dewaxing and rehydration → antigen retrieval → blocking → primary antibody → secondary antibody → chromogenic development (IHC) or direct imaging (IF) → counterstaining and mounting.

Two steps affect results most:

• Antigen retrieval: fixation masks some epitopes, so heat or enzymatic treatment is needed to expose them; too little or too much retrieval weakens the signal;

• Blocking: blocking non-specific binding sites first reduces background; otherwise the whole slide looks dark and is hard to interpret.

5. What to watch for on a fluorescence microscope

• Autofluorescence: some tissues (elastic fibres, red blood cells) emit light themselves and can be mistaken for signal — use controls to distinguish;

• Background and exposure: overexposure makes strong signals "bloom"; underexposure hides weak ones — adjust channel by channel;

• Photobleaching: fluorescence fades under illumination, so capture images within a reasonable time;

• Channel bleed-through: overlapping spectra of different fluorophores interfere with each other, so leave enough separation when choosing fluorophores and filters.

Conclusion

What IHC and IF do is translate information at the molecular level into images visible under a microscope. The first suits archiving and routine pathology; the second excels at multicolour and quantification. Using them well starts with understanding how antibodies, labels and the microscope work together.