Substrates of cytochemical reactions in this immunostaining
It's hard to understand the question, but in any immunocytochemical staining such as the above, you have two different types of reactions:


the antibody binding to the target (in this case, some laminin) 
the peroxidase-based colorimetric reaction with DAB. DAB (3,3'-diaminobenzidine tetrahydrochloride) is oxidized in the presence of hydrogen peroxide to form a brown precipitate, which becomes the stain.


The fact that is an indirect immunostaining implies that you have a non-conjugated primary antibody against laminin and a secondary antibody against the Fc part of immunoglobulins of the species in which the first antibody was raised. This secondary antibody will be conjugated with HRP (horseradish peroxidase, the enzyme responsible of generating the peroxides that will oxidize the DAB).

For a more complete reference of immunocytochemical stainings, you can read: http://www.ihcworld.com/_books/Dako_Handbook.pdf