The present invention relates to a set of polypeptides and its uses. In particular, the present invention relates to a set of polypeptides whereby this set comprises two polypeptides each of which comprises a targeting moiety “T” binding to an antigen “Λ” and a fragment of “F” of a functional domain, wherein said two polypeptides are not associated with each other in absence of a substrate that has “A” at (on) its surface and wherein, upon dimerization of “F”, the resulting dimer becomes functional. Furthermore, medical and diagnostic uses of said set are described. Moreover, the present invention relates to nucleic acid molecule(s) encoding said set of polypeptides. The present invention also relates to a vector comprising the nucleotide sequence of nucleic acid molecule(s) encoding said set of polypeptides. Furthermore, the present invention relates to pharmaceutical compositions comprising said set of polypeptides. Moreover, the present invention relates to a kit comprising said set of polypeptides.