Affinity imaging of red blood cells using an atomic force microscope

We used an atomic force microscope (AFM) to produce an image of a mixed lay er of group A and O red blood cells with a contrast based only on the measu red strength of a specific receptor-ligand pair. The image was obtained by measuring and plotting for each image pixel the adhesion force between the mixed RBC layer and the AFM tip functionalized with Helix pomatia lectin. T he high specificity of that lectin for the N-acetylgalactosamine-terminated glycolipids present in the membrane of group A RBCs enabled us to discrimi nate between the two cell populations and to produce an image based on affi nity contrast. The rupture force of the adhesion events leading to the imag e formation were quantitatively analyzed and compared to rupture forces mea sured with the same AFM tip on N-acetylgalactosamine tethered to agarose be ads. The mean rupture force was found to be 65 pN when measured on the grou p A RBCs and 35 pN on the agarose beads. These results show that the adhesi on, mediated by only a few receptor-ligand pairs, produces sufficient contr ast for the affinity image formation.