There is provided a method of forming a multimeric complex having affinity for a target. The method comprises: obtaining a plurality of self-assembly molecules, said self-assembly molecules including complementary self-assembly units such as verotoxin subunit B, each of which is operatively connected to an interaction domain such as a single domain antibody specific for the target; and combining said self-assembly molecules such that at least three said self-assembly units simultaneously bind to one another so as to permit the single domain antibodies to bind the target.