Affinity purified Urtica dioica agglutinin (UDA) is isolated from the rhizome of the stinging nettle. UDA differs from other known plant lectins with its molecular structure composed of only a single polypeptide chain, in contrast to other lectins composed of two subunits. It is composed of 86-amino acid residues and has a molecular weight of 8,500. It possesses extremely low specific agglutination activity regardless of blood type and has a carbohydrate specificity for GlcNAc. UDA is composed of several isoforms and has only homology to the chitin-binding domains of other proteins, specifically WGA. It is stable as it retains its activity even when subjected to acid conditions of pH 1.0, or heated to 80°C for up to 15 minutes.UDA is a T cell mitogen distinguishable from classical T cell lectin mitogens by its ability to discriminate a particular population of CD4+ and CD8+ T cells, as well as its capacity to induce an original pattern of T cell activation and cytokine production. T cell proliferation induced by UDA is dependent on MHC class II molecules but is not MHC restricted. UDA binds to specific carbohydrate structures present on class II molecules. It has been proposed that this lectin is a superantigen and could provide a useful probe for the analysis of T cell activation by superantigens. This lectin has also been shown to possess both antifungal and insecticidal activities.