🧠 Introduction: Why display antibodies?

Monoclonal antibodies (mAbs) are blockbuster drugs — 💊 Humira alone made billions. These powerful Y-shaped proteins fight diseases from cancer to inflammation. To find them, we need antibody display technologies, which link a protein (phenotype) to its DNA (genotype). This lets us screen billions of variants for the best “fit” against a target — all in vitro!

⚙️ Antibody structure & displayable formats

• The main antibody type is IgG — two heavy + two light chains = Y-shape.
• Fab = antigen-binding fragment; Fc = constant region for immune functions.
• Full IgGs are tricky to display in bacteria, so we use smaller fragments:
  • scFv: VH + VL + linker
  • Fab: VH–CH1 + VL–CL
  • sdAb or nanobody: single domain from camels 🐪 or sharks 🦈 — small, stable, easy to engineer.

🧬 Sources of antibody diversity

To build huge antibody libraries, scientists mix or design DNA sequences:

• Immune libraries – from immunized animals or patients; already affinity-matured but target-specific.
• Naïve libraries – from non-immunized donors (natural repertoire).
• Synthetic / semi-synthetic libraries – lab-made with controlled CDR loop designs, avoiding self-tolerance. These libraries can hold 10⁹–10¹² unique clones! 🧫

🦠 Phage display (PD)

The superstar of display tech 🌟 Phages (M13 viruses) show antibody fragments on their surface while carrying the DNA inside. Using “panning” (like gold mining 🪙):

1. Incubate phage library with immobilized antigen.
2. Wash away weak binders.
3. Elute and amplify strong ones.
4. Repeat 2–5 × until only high-affinity winners remain.

✅ Advantages:

• Enormous libraries (up to 10¹¹).
• Easy selection and mutation for affinity maturation, humanization, or bispecific design. 📈 Success: 14 marketed antibodies (e.g. Adalimumab = Humira, Atezolizumab = Tecentriq).

🧫 Bacterial display

Bacteria like E. coli present antibodies on their membranes. Pros: fast growth, cheap culturing, FACS-compatible. Systems include:

• Lpp-OmpA outer-membrane fusion.
• APEx and MAD-TRAP for inner-membrane display.
• Gram-positive S. xylosus and S. carnosus for surface anchoring via sortase. Limitations: fewer post-translational modifications and lower transformation efficiency than yeast/mammals.

🍞 Yeast surface display (YSD)

A eukaryotic upgrade using Saccharomyces cerevisiae 🧁

• Uses Aga1p–Aga2p fusion to anchor antibodies on the cell wall.
• Compatible with FACS, allowing real-time selection for affinity + expression.
• Ideal for affinity maturation, humanization, or creating pH-responsive and bispecific Abs. Typical library size ≈ 10⁹ — smaller than phage but allows exquisite control 🎯 Bonus: better protein folding quality due to eukaryotic machinery.

🧍 Mammalian display

The high-fidelity platform 🧫➡️🧍‍♂️

• Displays full-length IgGs on mammalian cell membranes using fusion to transmembrane domains.
• Can also secrete the same antibody for testing (“display + secretion”).
• Modern methods use transposons or CRISPR/Cas9 for stable, single-copy insertions.
• Enables selection for developability, proper folding, and human-like glycosylation.
• Slower growth = smaller libraries (10⁷–10⁹), but the results are closest to clinical production conditions.

🧪 Ribosome display (RD)

A cell-free system — no cells, no limits 🚫🧫

• Translation stops before the stop codon → forms an mRNA–ribosome–protein complex.
• This links genotype and phenotype directly.
• Libraries up to 10¹⁵ variants!
• Repeated selection cycles + RT-PCR recover top binders.
• Useful for affinity maturation and evolving antibodies entirely in vitro.

🧫 B cell selection

The most natural display system of all 💉

• Each B cell displays its own antibody and holds the matching gene.
• By isolating single B cells (via FACS + fluorescent antigens) and sequencing their Ig genes, we can clone fully human mAbs directly — preserving the natural heavy/light chain pairing.
• Microfluidic droplet systems now automate this process, allowing rapid screening of secreted antibodies for function and binding. 💡 Used to identify potent antibodies against HIV, Ebola, and COVID-19.

🧩 Conclusion

All display systems aim to link antibody function to its gene for selection and optimization: