CD22: An Immunoglobilin Superfamily Adhesion Molecule That Displays Sialic Acid-Binding Lectin Activity

CD22 is a B lymphocyte-specific adhesion molecule expressed on mature B cells. Recent work has shown that although CD22 has the structure of an immunoglobulin super-family member, its functional properties are those of a sialic acid binding lectin. The uniqueness of CD22 is underscored by the observation that it recognizes sialic acid in a specific linkage only and that sialylation of CD22 itself by α2,6 sialyltransferase can abrogate its function as an adhesion molecule. The discovery that the leukocyte-specific phosphoty-rosine phosphatase, CD45, is among sialoglycoproteins recognized by CD22, adds to the potential interest of CD22 as a molecule that may participate in the regulation of lymphocyte activation. This review will summarize our current understanding of the CD22 molecule and its putative functions.