The present invention is directed to a method of controlling and directing cells, for example to stimulate an immune response, inhibit an immune response, direct tissue regeneration or prevent tissue damage for therapeutic activity through the use of heterodimerically-tethered bispecific protein complex of formula A-X:Y-B. Component A may present X on the surface of a cell, may bind a protein (including a marker) expressed on the surface of an effector cell, or A-X is expressed on the surface of an effector cell, whilst B is specific to an epitope on target cell or tissue of interest. X:Y is a heterodimeric-tether which is formed by a binding interaction between X and Y, which together with A and B assists and effects the controlling and directing of the selected cells.