Intracellular staining and Fc block

Fc receptors bind the tail of an antibody. Intracellular staining opens the cell so that tail, and the dye, can stick in places the antigen never was. The decision this guide supports is where to put an Fc block, a viability dye and the intracellular antibody so a positive gate is more than stickiness. Scoring the resulting events is described in how a flow cytometer reads a cell. Localising the same stain in space is part of fluorescence microscopy without the myths.

Platforms are listed in the scientific instruments catalogue. The assay sits in the molecular biology pathway. Channel needs for the viability dye and the intracellular fluorophore go in a quote request.

Who the block is for

Fc receptors are a family of surface proteins that bind the constant region of immunoglobulin. Monocytes, macrophages, dendritic cells and granulocytes are rich in them. B cells and natural killer cells express members of the family too. Mouse spleen and human blood are not interchangeable: the reagent that blocks one species may not bind the receptors of the other. The sheet of the block reagent names the species. A "universal" block that does not name your species is a reagent you have not yet tested.

If the parent gate is a pure population you already know lacks Fc receptors, the block changes little. If the parent is mixed tissue, the block is how you stop myeloid cells from impersonating your rare marker. Write the parent down. The decision follows from it.

What the block is, as a reagent class

Typical blocks are purified immunoglobulin of the right species, an antibody directed at the receptor itself, such as an anti-CD16/CD32 class reagent in the mouse, or a commercial Fc-block formulation. The mechanism you care about is occupancy: the receptor is full before the staining antibody arrives, so the staining antibody has to bind by its antigen-binding site. Adding the block after the stain does not reliably peel off antibody that has already bound.

Serum in the stain buffer is sometimes described as a block. It can reduce some nonspecific binding and it is not automatically an Fc-receptor occupancy step for your species. If the method relies on Fc block, name the reagent. Do not assume fetal calf serum did the job.

Intracellular buffer proteins, such as serum or albumin in a permeabilisation wash, reduce stickiness inside the opened cell. They sit alongside Fc block. They do not replace a surface block that should have happened earlier. Dead cells remain sticky even in a rich buffer. A fixable viability dye, applied before fixation, is the gate that removes them.

Order of an intracellular experiment

Start with the biology. A cytokine stain usually needs the cells to have been cultured with a protein-transport block so the cytokine accumulates. That culture step is not the Fc block. Do not mix the names. After culture, stain a fixable viability dye in the buffer its sheet requires, often protein-free. Then Fc-block. Then surface antibodies. Then fix with the formaldehyde class the intracellular clone accepts. Then permeabilise with the detergent or alcohol class the clone accepts. Then stain the intracellular antibody in a buffer that keeps the pores open. Times and concentrations stay on the sheets you actually used.

For a nuclear factor, the permeabilisation class is often harsher. Surface stains that cannot survive that class must go on first, and their fluorophores must be ones the later alcohol or strong detergent does not destroy. If you cannot satisfy both, split the panel or change clones. Forcing one buffer to serve two incompatible clones produces a tidy protocol and a false negative.

On a microscope the same order applies to cells on a coverslip, with extra attention to aldehyde background. An unstained coverslip, fixed and permeabilised, sets the exposure floor. The MicroscopyU fluorescence section explains why that floor is optical as well as chemical.

Branch when the blocked tube still looks dirty

Run one clone you know can bind Fc, with and without block, on the same cells. If the block lowers that signal, the reagent works, and a remaining intracellular signal is more credible. If the block changes nothing, the reagent may be the wrong species, expired, or too dilute relative to its own sheet. Increase only within the sheet's range. A huge excess of the wrong species will not occupy the receptor.

If the intracellular channel is bright on a fluorescence-minus-one tube that still contains the viability dye and the surface panel, you are looking at spill or spreading. Compensate the viability dye from a single stain at least as bright as the dead cells. If the fluorescence-minus-one tube is clean and the fully stained tube is bright only in the viability-positive cells, gate those cells out. If live cells remain positive against a biological negative, you have a result worth repeating. If there is no biological negative, say that the gate is relative to fluorescence-minus-one only.

Failure modes

A block reagent that contains a fluorophore, or that is contaminated with a fluorescent antibody from a shared tray, becomes an extra colour. Run it alone once. A block that is an antibody against the receptor can itself be bound by secondary reagents. If your intracellular step uses a secondary antibody, the block must not be a target of that secondary. Primary conjugates avoid that trap. Secondaries need a plan.

Saponin washed out before the intracellular stain closes the path and looks like a negative experiment. Keep the detergent in the buffer when the sheet says the pores are reversible. Alcohol protocols have the opposite hazard: they strip the Fc block's context and some surface fluorophores. Read the intracellular sheet again when you change perm class.

Safety and research limits

Fc block and intracellular stain do not decide containment. Unfixed human material remains an institutional decision under the WHO Laboratory biosafety manual and the CDC BMBL. Fixation is not automatically accepted inactivation. This guide does not approve a sample and does not describe a diagnostic immune panel. Method reporting in cytometry is part of the conversation at the International Society for Advancement of Cytometry.

What to send with an enquiry

State the species, whether myeloid cells are in the sample, the intracellular target class, and the detectors you need for viability plus the intracellular dye. Ask for filter bands. Use the scientific instruments catalogue and the quote request. Ask whether a quotation is possible. A cytometer name does not choose the Fc-block reagent.