What does the Protein A capture step do in antibody purification?

Why

Clarified harvest contains the antibody at grams per liter alongside thousands of host cell proteins, DNA, media components and cell debris. Protein A, a bacterial protein that binds IgG Fc, is immobilized on a resin. A 2010 review of antibody purification describes it as giving a high degree of purity and recovery in a single step, with dynamic binding capacities of roughly 15 to 50 g of antibody per liter of resin and elution at pH 2.5 to 4.

This is why almost every antibody platform starts with it. The low-pH eluate also feeds naturally into a viral inactivation hold, and ICH Q5A(R2) notes experience with efficient inactivation of murine leukemia virus and pseudorabies virus by low-pH incubation after Protein A capture. The step has limits.

Protein A ligand leaches from the resin and must be removed by later polishing steps. Resin is expensive, so its lifetime and number of cycles matter, and viral clearance across reused resin needs support. Low pH can trigger aggregation of sensitive antibodies.

Some host cell proteins bind to the antibody itself and co-elute, which is why polishing steps such as ion exchange remove residual host cell protein, DNA, leached Protein A, aggregates and viruses. Fc-fusion proteins usually bind too, while antibody fragments without Fc need other capture methods.