What is the key substance found in the developing buffer?

Enzyme linked to the secondary antibody   
an undetermined measure of antigen is joined to a surface, and after that a particular counter acting agent is connected over the surface with the goal that it can tie to the antigen. This counter acting agent is connected to a catalyst, and in the last stage a substance is included that the compound can change over to some distinguishable flag, most normally a coloration change in a concoction substrate is seen.