Purified CD4 cells were labeled with AlexaFluor-488-conjugated anti-CD4 antibodies, introduced to the flow cell in amounts ranging from zero to 200,000 cells and imaged. There is a linear correlation between the number of cells in the sample and the intensity of light emitted from the membrane filter (R² = 0.999).

From: Rodriguez WR, Christodoulides N, Floriano PN, Graham S, Mohanty S, et al. (2005) A Microchip CD4 Counting Method for HIV Monitoring in Resource-Poor Settings. PLoS Med 2(7): e182