A myeloid panel fails for two reasons that look the same on the plot: a marker that is genuinely absent, and a marker that was there until the sample was handled. Several of the most useful surface proteins on these cells are shed by enzymatic dissociation, moved by activation or expressed across more than one lineage, so a panel assembled from a marker list without regard to the preparation will separate populations that are artefacts of the protocol. Reading the panel by lineage first, and then by what the protocol does to each marker, is the difference between a clean gate and a plausible one.
Which neutrophil markers actually gate the population
Granulocytes are usually taken on a combination rather than one protein: a granulocyte specific surface marker, a marker of the myeloid lineage as a whole, and a scatter profile that separates them from monocytes. In mouse work the two commonly used granulocyte and monocyte discriminators cross react with each other's populations to a degree that matters, so they are read as a pair rather than separately. Whichever combination is used, include a marker that is not expected to move, because an activated granulocyte changes several of them at once.
Macrophage markers, and why one macrophage marker is never enough
Tissue macrophages are defined by a set rather than a protein, and the set differs by tissue: a scavenger receptor, a pan macrophage surface or lysosomal protein and a lineage transcription factor are typically combined, and each of them is present on some dendritic cells too. Polarisation state changes the useful markers as much as the tissue does, so a panel built to show a phenotype should include markers on both sides of it rather than only the one expected. Intracellular members of the set need fixation and permeabilisation, which is a different buffer set and has to be validated with the surface stain rather than after it.
Microglia marker choices, and the microglia markers that separate them from monocytes
The long standing problem here is that the classical markers are shared with infiltrating monocytes and macrophages, so a marker that labels microglia beautifully in a healthy brain labels both populations in an injured one. The purinergic and transmembrane proteins identified over the last decade are the ones that discriminate, and they are sensitive: expression falls in culture and after prolonged dissociation, so a preparation that took hours reports fewer microglia than it contains. Dissociate cold and quickly, and include one marker that is not activation dependent.
Where CD66b marker staining fits a granulocyte panel
This is a granulocyte activation marker rather than a lineage marker, and that is the point of including it: it appears on the surface when secondary granules mobilise, which makes it a readout of the state of the cell rather than of its identity. It is therefore useful beside a lineage marker and misleading instead of one. The same caution applies to every activation marker on these cells, because a sample that sat on the bench activates on its own and the plot will show it.
Surface or intracellular: which cell surface markers survive dissociation, and which cell surface marker sets do not
Enzymatic dissociation cleaves some of the most useful epitopes and leaves others untouched, and the affected list is specific to the enzyme rather than general. A marker that reads as absent in a tissue preparation and present in blood from the same animal is usually a cleaved epitope rather than a biological difference. Where a panel must include a sensitive marker, the choices are a gentler dissociation, an antibody clone raised against a different epitope, or reading that marker only in preparations where it is reliable and saying so.
Assembling the panel in the order that catches mistakes
Start from the population you need to resolve rather than the markers available, write down which of them are lineage and which are state, and assign the brightest conjugates to whichever markers are scarcest on the cells you care about. Then stain a sample you already understand, deliberately including a control that should be negative, because a panel that has never been shown to produce a negative result has not been tested. Keep one combination across a study; a marker swapped halfway through makes the two halves incomparable whatever the gating says.
Questions people ask about myeloid markers
Is one pan myeloid marker enough to gate on?
No, because every candidate is either expressed outside the lineage or moved by activation. Two markers of different kinds, one of lineage and one that is stable, plus the scatter profile, is the usual minimum, and in tissue it is worth adding a viability dye because dying cells bind antibodies non specifically.
Do mouse and human panels translate?
Partly, and the gaps are where mistakes happen. Several of the discriminating proteins in one species have no clean counterpart in the other, and some antibodies that read well in blood read poorly in tissue. Build the panel for the species and the tissue rather than translating a published one.
Why does my tissue preparation lose a marker the blood sample shows?
Most often the epitope was cleaved by the dissociation enzyme. Compare the same marker in a mechanically dissociated aliquot, or choose a clone against a different epitope; if neither is possible, report that marker only where it is reliable.