https://phys.org/news/2018-06-molecular-method-cancer-therapy-diagnostics.html

Around 1900, Paul Ehrlich, the winner of the 1908 Nobel Prize in physiology or medicine, envisioned drugs that would have a component that recognizes pathogens in the body and another component that acts on the target. Usually, such drugs target receptors on the surface of the damaged cells. These receptors also allow the agent to recognize the cell. A universal system of this kind can be used for diagnostics, therapy, or both. When therapy and diagnostics are combined, this is known as theranostics.

To make a system incorporating a therapeutic and a diagnostic component, a "molecular glue" is needed for holding the two parts together. This glue can be realized as proteins capable of forming a stable complex by binding to each other. One of the most stable complexes of this kind is the barnase-barstar protein pair. There is a value known as the binding constant that characterizes how strongly the molecules in a complex are coupled. The binding constant of the barnase-barstar complex is 1,000 to 1 million times greater than those of the antigen-antibody complexes, which are the basis of our immune response.