The B cell surface molecule CD22 is a member of the Siglec family. Siglecs possess a conserved membrane-distal immunoglobulin domain that mediates binding to sialylated glycoproteins or glycolipids. Although the structural basis of sialic acid recognition by Siglecs is quite well understood, the binding properties of the interaction between Siglecs and their native ligands have not been investigated. CD22 binding requires alpha2-6-linked sialic acid, which is mostly carried on N-glycans. One protein that carries such N-glycans is CD45. In this study we used surface plasmon resonance to perform thermodynamic and kinetic analysis of CD22 binding to native CD45. CD22 bound with a low affinity (K(d) 130 microM at 25 degrees C) and very fast kinetics (k(off) >or=18 s(-1), calculated k(on) >or=1.5 x 10(5) M(-1)s(-1)). Van't Hoff analysis revealed that binding was enthalpically driven at physiological temperatures, as is typical of most lectin-carbohydrate interactions. Since there is evidence that CD22 binds preferably to CD45, even though many cell surface proteins carry alpha2-6-linked sialic acid, we compared the affinities of CD22 binding to CD45, to CD4 carrying alpha2-6-linked sialic acid, and to a synthetic alpha2-6-sialoglycoconjugate. The affinities did not differ substantially, suggesting that CD22 binds preferentially to CD45 not because the latter presents higher affinity ligands but because it carries multiple copies of thereof.