--CH2 F, and --CH2 NR3 2 wherein each R3 is independently an alkyl of 1 to 5 carbon atoms; and ##STR41## wherein Me is a methyl group, R1, R2, and R3 are each independently --OH, --F, and --NRR wherein each R is independently hydrogen or an alkyl of 1 to 5 carbon atoms; and contacting the sample with the surface of the substrate; and determining the presence of selectin receptors bound to the s