Cell Surface Protein Internalization and Loss (Flow Cytometry (Troubleshooting))

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Expected cell surface markers show weak or absent staining despite known expression in the cell type. Loss of fluorescence intensity occurs specifically for membrane proteins, while intracellular markers remain detectable. Common causes: 1) Cell surface proteins internalized due to sample processing at room temperature or above instead of 4°C 2) Trypsin treatment during cell detachment inducing internalization of surface molecules, particularly cell surface receptors 3) Surface antigen modulation and internalization occurring without sodium azide to prevent endocytosis Solutions: 1) Perform all staining steps on ice or at 4°C using ice-cold reagents to stop metabolic reactions and prevent internalization 2) Use gentle detachment methods instead of trypsin for cell lines; try cell scrapers, EDTA-based dissociation, or enzyme-free dissociation buffers to preserve surface proteins 3) Add sodium azide to buffers to prevent modulation and internalization of surface antigens; work quickly to minimize time at non-refrigerated temperatures