Protein purification processes consist of several steps. Optimally, an affinity chromatography resin is available, such as Protein A for immunoglobulins, since it results in very pure target protein. One or two polishing steps follow, which can be ion exchangers, hydrophobic interaction or mixed mode resins. Often the buffers have to be exchanged or adapted between steps.
The goal of each downstream process is to get rid of host-cell proteins and DNA, eliminate possible virus contamination, and deplete aggregated or inactive target protein. Preserving functionality must always be kept in mind.