A polyclonal antiserum recognizes more than the protein you raised it against. Some of that extra reactivity never shows up. Some of it lights up the wrong band on a Western blot or stains every cell in a tissue section. Pre-adsorbing the antibody against an acetone powder is the old, cheap way to pull that background out before you run the experiment, and it still works.
Why it works
An acetone powder is tissue that has been homogenized, washed repeatedly in cold acetone to remove water and lipid, and dried. What's left is a stable, insoluble matrix of the tissue's proteins with their antigenic sites mostly intact. Stir an antibody solution through it and any antibody that recognizes those proteins sticks to the powder. Spin the powder down and the supernatant has lost exactly the reactivities you didn't want.
The trick is picking a powder from a tissue that carries the off-target antigens but not your target. If the antibody was raised against a muscle protein, a powder from a tissue that lacks that protein removes the shared background and leaves the specific antibody alone. Most labs settle on one of a few standard powders and validate the result rather than reasoning it out from first principles.
A protocol that works
Pick the powder to match the background you want to remove. Rabbit thymus and rabbit brain acetone powder are the usual general-purpose choices. For antibodies against cytoskeletal and muscle proteins, use rabbit muscle acetone powder.
Add roughly 1% (w/v) powder to the diluted antibody and mix gently at 4 °C. An hour is a reasonable starting point; go overnight for a stubborn background.
Centrifuge hard enough to pellet the powder completely, then pass the supernatant through a 0.45 µm filter so no particles carry over into the assay.
Then check the result by running the pre-adsorbed and untreated antibody side by side on the same blot or section. If the specific signal drops along with the background, the powder you chose contains your target, so try another tissue.
What it will and won't do
Pre-adsorption removes cross-reactive antibody; it doesn't raise affinity, and it won't rescue a poorly characterized antiserum. It earns its keep in immunohistochemistry and Western blotting with polyclonal sera, especially when the secondary antibody or the sample species shares epitopes with the immunogen. The powder keeps for years dry at −20 °C, so one jar covers a lot of experiments.
We prepare our acetone powders from tissue harvested in house, so the source, age and health status of the animal are known for every lot. If you're deciding between tissues for a particular antibody, ask us and we'll point you to the powder other customers use for that target. For how the powders are made and stored, see acetone powders: preparation, attributes and uses.