Hmm this is going to be challenging. So Human serum with anti-human antibodies in it? How does that work, are these autoantibodies?
The background issue is because your anti-human secondary is binding to all of the IgG that are innate to tissue. Its not perfused, so all those IgGs are fixed right in there and since the DRG seems more vascularized in human than mouse, its probably an awful amount of background!
In mouse, the mouse antibody issue is a problem if you are using non-perfused highly-vascularized tissues. That’s why most people perfuse, to get rid of all the IgGs in the blood… in brain it can be a problem, but its not as noticeable in the DRG. If I am using non-perfused mouse tissue, I cut the background down by using isotype specific anti-mouse secondaries since most mouse primary antibodies are monoclonal. But you cant do that for human since I assume your preps are polyclonal.
That kit from Vector labs seems promising - looks like they “block” all the innate IgGs with some type of human primary. You have to use immunoperoxidase staining though so you are limited to just one primary.