The HIB22 antibody reacts with CD22 (Siglec-2), a type I transmembrane glycoprotein expressed in the cytoplasm of pro-B and pre-B cells and on the surface of mature and activated B cells and B cell lymphomas, but not on plasma cells. CD22 functions as an inhibitory receptor for B cell receptor signaling and plays roles in modulating the activation threshold of B cells and controlling CD40 signaling. CD22 comprises two isoforms generated by alternative splicing: CD22α (~130 kDa) and CD22β (~140 kDa). Monomeric CD22β is the most abundant isoform but an αβ heterodimer is also expressed. CD22 is a member of the sialoadhesion subgroup within the immunoglobulin superfamily, with an N-terminal V-type Ig domain followed by six C2-type Ig domains. Binding of the HIB22 antibody reportedly requires the presence of the two N-terminal-most domains. Both the cytoplasmic and surface-bound forms of CD22 are recognized by the HIB22 antibody and its binding does not inhibit CD22-mediated cyto-adhesion.