Mature human RBCs lack expression of the coreceptor for parvovirus B19 and do not internalize the virus. (A) Flow cytometric analyses of surface expression of P antigen (left), α5 (middle), and β1 integrins (right) in pooled normal donor RBCs. Control cells stained with secondary antibodies only are shown in black. (B) Confocal microscopy of RBCs with bound Cy3-labeled (red) parvovirus B19 (Control; i) and that following treatment with trypsin (+Trypsin; ii). Image acquisition was achieved using a Zeiss confocal microscope detecting red fluorescence in the presence of transmission light to visualize cell contour/morphology.