Order the steps that recover and purify a biological product and explain what each removes.
Why it matters
At the end of a production run the product, for example a monoclonal antibody, is a tiny fraction of what is in the bioreactor: cells, cell debris, host-cell proteins, DNA, media components, and possibly viruses. Downstream processing turns that mixture into a pure, safe, concentrated drug substance. It is often the most expensive part of manufacturing, and every step costs yield. Technicians run chromatography, filtration and buffer preparation, and need to know what each step is for.
The usual order
Downstream processing usually follows this logic:
- Harvest and clarification: remove cells and debris.
- Capture: grab the product and concentrate it, removing most impurities in one step.
- Virus inactivation: chemically or physically kill enveloped viruses.
- Polishing: remove remaining trace impurities, aggregates and variants.
- Virus filtration: physically remove viruses by size.
- Concentration and buffer exchange: bring the product to final concentration in its formulation buffer.
- Sterile filtration and fill: filter through 0.2 um filters into containers for storage or further processing.
A typical monoclonal antibody (mAb) platform looks like this:
| Step | Common technology | Removes or achieves |
|---|---|---|
| Clarification | Centrifugation and/or depth filtration | Cells, debris |
| Capture | Protein A affinity chromatography | Most host-cell proteins, DNA, media; concentrates the mAb |
| Virus inactivation | Low-pH hold (for example around pH 3.5 to 3.7 for a set time) | Enveloped viruses |
| Polishing | Cation and/or anion exchange chromatography (sometimes hydrophobic interaction) | Aggregates, residual host-cell proteins and DNA, leached Protein A, variants |
| Virus filtration | Nanofiltration (pores around 20 nm) | Viruses, including small non-enveloped ones |
| UF/DF | Ultrafiltration/diafiltration by tangential flow filtration (TFF) | Concentrates product; exchanges into formulation buffer |
| Final filtration | 0.2 um sterilising-grade filter | Bacteria |
Products made in bacteria differ: E. coli often makes proteins inside the cell, sometimes as insoluble inclusion bodies, so downstream starts with cell lysis and may include protein refolding.
How chromatography separates
- Affinity: a ligand binds the product specifically (Protein A binds the Fc region of antibodies). Impurities flow through; the product is eluted, often with low pH.
- Ion exchange: proteins bind according to their charge at a given pH, and are eluted by increasing salt or changing pH. Depending on conditions, the product either binds while impurities flow through (bind and elute), or the impurities bind while the product flows through (flow-through mode).
- Hydrophobic interaction and size exclusion separate by surface hydrophobicity and size.
Yield adds up
Each step loses some product, and losses multiply.
Worked example. Six steps with yields of 95, 90, 98, 92, 95 and 97 percent:
overall yield = 0.95 x 0.90 x 0.98 x 0.92 x 0.95 x 0.97 = about 0.71 (71 percent)
Removing or improving even one step can matter a great deal at manufacturing scale. Process development balances purity against yield and cost.
What quality checks look for
In-process and release testing measure product concentration, purity (often by size-exclusion HPLC and capillary electrophoresis), host-cell protein and host-cell DNA levels, aggregates, endotoxin, bioburden, and potency. Virus clearance is demonstrated by spiking studies during validation, not tested on every batch. Specifications for these tests are defined in the product's regulatory filing, guided by international guidelines such as ICH Q6B for biotechnological products (ICH quality guidelines: https://www.ich.org/page/quality-guidelines, checked on 2026-10-23).
Common mistakes
- Thinking of purification as one step rather than orthogonal steps that each remove different impurities.
- Forgetting the cumulative effect of small yield losses.
- Treating a low-pH hold as optional when it is a validated virus-inactivation step with set time and pH.
- Confusing ultrafiltration (concentrates product) with sterile filtration (removes bacteria).
- Holding product between steps longer than validated hold times.
Quick recap
- Clarify, capture, inactivate viruses, polish, filter viruses, UF/DF, sterile filter.
- mAb platform: Protein A capture, low-pH hold, ion exchange polishing, nanofiltration, TFF, 0.2 um filtration.
- Each step removes different impurities; yields multiply.
- Quality tests cover purity, HCP, DNA, aggregates, endotoxin and potency.
Try it
The Do tab has a quiz on ordering steps and what each removes.