Antibody purification routes compared: where protein A, protein G and protein affinity chromatography each belong, what an anti protein a antibody is for when leached ligand has to be measured, how protein a chromatography for antibody purification scales, what monoclonal antibody purification adds, which protein purification buffer the eluate needs, and the polishing step that removes aggregate

Designing the purification

1. Capture on protein A or protein G, chosen by species and subclass. Protein A binds many immunoglobulin G subclasses well and some poorly; protein G has a broader subclass range and binds some species where protein A does not. Check both against the species and subclass you have rather than defaulting, because a poor match halves the yield before anything else happens.
2. Use antigen affinity when specificity is the product. Protein A or G gives total immunoglobulin, including everything the animal made against everything else. Passing it over immobilised antigen gives the specific fraction, which is what a polyclonal reagent usually needs and what a monoclonal does not.
3. Neutralise the eluate immediately. Affinity elution is at low pH, and time at low pH is what denatures and aggregates antibodies. Collect into a neutralising buffer already in the tube rather than neutralising afterwards, and keep the fractions small.
4. Polish on size exclusion to remove aggregate. Aggregate causes high background in every application and is invisible on a reducing gel. A size exclusion step both removes it and tells you how much there was, which is a quality measurement worth keeping for each preparation.
5. Formulate for storage, not for the column. The final buffer decides shelf life. A neutral buffered saline with a carrier protein and a cryoprotectant suits most reagents; sodium azide preserves but interferes with peroxidase detection, which is a conflict worth resolving before the aliquots are made.

Measuring what you got

Concentration by absorbance with the correct extinction coefficient, purity on a reducing and a non-reducing gel, aggregate by size exclusion, and activity by the assay the reagent is for. Three of those four are routinely skipped and the fourth is the only one that matters to the user.

Keep the numbers with the aliquots. A reagent whose concentration is remembered rather than recorded produces experiments whose antibody amount is unknown.

Scale and when to outsource

Below a few tens of milligrams, a gravity column and an afternoon is usually the right answer. Above that, a chromatography system pays for itself in consistency, and beyond a gram or so a contract provider with validated columns is generally cheaper than acquiring the capability.

If outsourcing, agree the release specification, who owns the residual and what happens to the resin. Antigen resin made for your project is an asset worth retaining.

Common failures and what they look like

Low yield with a clean gel usually means poor resin binding for that subclass. Good yield with a smear at high molecular weight on a non-reducing gel is aggregate. A reagent that works on the day and fails a month later is a formulation problem, not a purification one.

Losing activity but not protein points at the elution. Shortening the acid exposure and increasing the neutralising capacity of the collection buffer is the first thing to change.

Common questions

Protein A or protein G for antibody purification?

Decide from the species and subclass. Protein G has broader subclass coverage for several species; protein A gives cleaner elution for others. For mixed subclasses, a protein A/G resin avoids losing part of the preparation.

How do I avoid aggregation during purification?

Minimise time at low pH by neutralising into the collection tube, avoid concentrating beyond what the application needs, and polish on size exclusion. Concentration steps cause more aggregate than elution does in most preparations.

Is antigen affinity purification always better?

For a polyclonal serum, usually, because it removes the antibodies raised against everything else. It also loses low-affinity specific antibodies, which occasionally matters, and it needs enough immobilised antigen to be worth doing.

What should the final buffer be?

Whatever is compatible with the application and the storage plan. Phosphate buffered saline with a stabiliser is a common default; check azide compatibility with your detection chemistry and whether the reagent will be conjugated later, because carrier protein blocks amine coupling.

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