A novel conjugate comprises a target seeking moiety and a modified wild type superantigen； the modified superantigen retains its ability to activate a subset of T cells, even though 1 or more wild-type amino acid residues in at least 1 region which functions in determining binding to T cell receptor (TCR)and activation of a subset of T cells has/have been replaced. Also claimed is the use of a modified superantigen as above, optionally as part of a conjugate with a target seeking moiety, for activating the immune system to treat a mammalian disease. Also new is a pharmaceutical composition comprising a modified antibody (preferably a Fab fragment fused to a peptide moiety providing activation of T cells in V~b specific manner) in which cysteines providing for interchain cysteine linkages in the native antibody have been replaced (preferably by serine residues) to prohibit cystine formation.
