N-acetyl-D-galactosamine specific lectin. Binds the Tn determinant (GalNAc-alpha-O-Ser/Thr) of the tumor-associated glycopeptide. Could be required for agglutinating cells such as Tn-exposed erythrocytes. Homo- or heterotetramer. V.villosa isolectins are composed of either two subunits a and two subunits B (A2B2), four subunits A (A4), or four subunits B (B4). The predominant form, isolectin B4, has no A1 erythrocyte agglutinating activity. Binds one manganese (or another transition metal) ion and one calcium ion. The metal ions are essential for the saccharide-binding and cell-agglutinating activities (By similarity). Belongs to the leguminous lectin family.