Fractionated Vicia villosa lectin or agglutinin (VVL/VVA) is a tetramer composed of four subunits. VVL consists of A and B subunits, with a dominant isolectin that appears to be B subunit-rich. A4 isolectin agglutinates specifically A1 erythrocytes. B4 isolectin does not agglutinate A, B, or O-erythrocytes; however, B4 binds the Tn antigen, a molecule expressed on the surface of most tumor cells. A2B2 and similar tetramer isolectins display properties characteristic of both A4 and B4 isolections.