Controls for autofluorescence

Autofluorescence is signal from the cell itself, and it sits under every antibody you add. The decision this troubleshooting note supports is whether a shift above the negative is a fluorophore or the cell's own glow, and which control is allowed to make that call. Pulses and gates are in how a flow cytometer reads a cell. The same glow in an image is part of fluorescence microscopy without the myths.

Instruments that collect it are in the scientific instruments catalogue. The assay may sit in the molecular biology pathway. Detector and filter choices belong in a quote request.

Where the glow comes from

Cells contain molecules that fluoresce without any stain you added. NAD(P)H and flavins contribute in the blue and green. Lipofuscin and other storage pigments contribute broader emission, especially in older or stressed cells. Granulocytes and macrophages are brighter than resting lymphocytes. Plants, algae and samples with chlorophyll bring their own strong bands. Collagen and elastin in tissue glow under a microscope. Aldehyde fixation adds more, particularly if the fixative is not fully washed out. Phenol red and riboflavin in culture medium can raise background if they travel into the tube.

The spectrum is broad compared with a good fluorophore. That is the practical clue. A stain that is "positive" in every detector at once, matching the unstained shape, is autofluorescence until proved otherwise. A stain that rises in its primary detector and nowhere else, against a matched unstained, is a fluorophore you can start to trust.

The control that matches

Acquire unstained cells at the same voltages or the same exposure as the experiment. They must be the same cell type. They must see the same fixation, permeabilisation, time on the bench and, for tissue, the same mounting. A cultured line's unstained tube does not set the gate for primary myeloid cells. An unfixed unstained does not set the gate for a fixed panel.

Plot the unstained cells in every channel you will gate. If two biological populations already differ in the unstained tube, they need separate boundaries or a marker that pulls them apart before you trust a dim fluorescence gate. A single vertical line across a mixed autofluorescent cloud will call the brighter lineage positive.

On the microscope, image the unstained slide through every cube, at the locked exposure. Note which cubes are hopeless. The MicroscopyU fluorescence section and the Evident fluorescence primer describe why emission windows collect this light. They do not subtract it for you.

Next checks when the unstained is already high

If the unstained cells sit where you hoped the positive stain would sit, the panel design is in trouble in that detector. Move the dim antigen to a fluorophore whose emission is farther from the worst autofluorescence, often away from the green on aldehyde-fixed myeloid cells, and confirm with a new unstained run. Raising voltage makes both the glow and the stain brighter. It does not improve the separation if both move together.

If only the stained tube is high and the unstained is low, you still need a single stain and, where the gate is close, a fluorescence-minus-one tube. Autofluorescence control does not replace compensation. Compensation does not replace the unstained tube. They answer different questions.

If a spectral cytometer or a spectral unmixing microscope is available, autofluorescence can sometimes be treated as its own component, using the unstained spectrum as the reference. That is a method class with its own rules: the reference must still match the sample's treatment, and a bad reference unmixes a ghost into every channel. Do not assume unmixing deletes the need to show the unstained data.

Failure modes that get published

Gating a dim marker on lymphocytes and then applying that gate to macrophages without an unstained macrophage tube calls autofluorescence a positive. Fixing samples and not the control does the same. Acquiring unstained cells at a lower voltage than the panel makes the background look solved. Media left in the well of a slide makes a microscope field glow in a way that disappears when you wash the well, which is a preparation artefact, not a cellular one.

Tandem dyes and bright dumps still matter. A cell can be both autofluorescent and spilling. The unstained tube will not show the spill. Look at both controls before you pick a cause. If they disagree with the story in the legend, change the legend.

Medium and mountant pretending to be cells

Culture medium is a frequent impostor. Phenol red and vitamin-rich supplements fluoresce, and a rinse that was optional for a bright antigen is required for a dim one. Spin the cells and resuspend them in the buffer you will acquire in, then rerun the unstained tube. If the glow drops, the cells were carrying medium. If the glow stays, it is cellular, and the detector choice still has to respect it. On a slide, an aged mountant or an incompatible antifade can add a veil that looks like tissue autofluorescence. Image a blank coverslip in that mountant at the same exposure. A bright blank means the reagent is the source. Replace it before you reinterpret the stain. These checks belong in the same notebook line as the voltages, because a later reader will otherwise treat the glow as a lineage.

Safety and research limits

Unstained cells are the same hazard as stained cells. Containment is institutional, with the WHO Laboratory biosafety manual as a reference. This note does not approve unfixed human tissue because it is "only a control". Autofluorescence is not a diagnostic marker in this article. Detector practice in cytometry is part of the conversation at the International Society for Advancement of Cytometry.

Warm rooms increase the dead-cell fraction, and dead cells autofluoresce more. If the unstained tube waited longer than the samples, its glow overstates the background. Match the wait, or stain and acquire in an order you record.

What to send with an enquiry

State the cell type, whether you must fix, and which emission bands you hope to use for dim markers. Ask which detectors are strongest away from the green autofluorescent region, and whether spectral unmixing is actually part of the instrument or only a slogan. Use the scientific instruments catalogue and the quote request. Ask whether a quotation is possible. A laser list without filter bands does not tell you where the glow will land.