High Signal in Negative Cell Populations (Flow Cytometry (CST Guide))

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Negative control populations (e.g., monocytes, unstained cells) show unexpectedly high fluorescence signal, reducing separation from true positive cells. Common causes: 1) Off-target cells (monocytes) express Fc surface receptors binding the Fc portion of primary or secondary antibodies, causing non-specific staining 2) Insufficient washing between antibody incubation steps leaving unbound antibody in suspension 3) Secondary antibody alone generating background signal independent of primary antibody Solutions: 1) Block cells with Bovine Serum Albumin, Fc receptor blocking reagents, or normal serum from same host as primary/secondary antibody prior to staining 2) Include secondary-antibody-only control to identify if background originates from primary or secondary antibody 3) Perform additional wash steps (3-5 washes) between antibody incubations to remove unbound reagents