Identification of cells based on membrane molecules by flow cytometry involves which labeling method?

• A. Fluorescent-labeled antibody
• B. Enzyme-labeled antibody
• C. Peripheral blood cells first separated by Ficoll-Paque
• D. Complement proteins attaching to target cell

Answer: (A)

Flow cytometry identifies cells by tagging their surface molecules with fluorescently labeled antibodies. An antibody specific for a membrane antigen binds to that marker on the cell, and the attached fluorochrome emits light when excited by a laser. By measuring the emitted fluorescence, often across multiple colors, you can determine which cells express which surface markers and thus identify cell types or subsets. This approach is inherently per-cell and multiplexable, so you can label several membrane proteins at once to define, for example, CD4 or CD8 status, B cell markers, or other phenotypes.

Enzyme-labeled antibodies aren’t the standard labeling method in flow cytometry because signals from enzyme reactions (like colorimetric changes) aren’t readily measured on a per-cell basis with the same ease or multiplexing capability as fluorescence. Separating peripheral blood cells by a density gradient (Ficoll-Paque) is a preparation step, not a labeling strategy. Complement proteins attaching to a target cell describe an immune process rather than a fluorescent tagging method used to identify cell surface markers in flow cytometry.