VHHs, also known as single-domain antibodies or nanobodies, are the smallest antigen-binding fragments derived from heavy-chain-only antibodies found in camelids, such as alpacas, llamas and camels. First discovered in 1993, VHH antibody fragments offer unique characteristics which are particularly advantageous in therapeutics, diagnostics, and research tools. This is due to their small size and structure which allows them to penetrate tissues and reach targets that may be challenging for conventional antibodies, in addition to their stability and ability to bind with high affinity to specific targets.1
There are several ways to isolate VHHs from camelids against a target of interest and to build a library for screening, such as immunized, naïve, and synthetic/semi-synthetic libraries. The most common approach is by immunizing camelids and building a library based on the repertoire of heavy-chain immunoglobulins. This process is central to VHH discovery, enabling the identification of high-affinity nanobodies for therapeutic or diagnostic applications.2,3 The process is as follows:
Camelid immunization: In 2 months, camelids are injected four to eight times with target antigens.
PBMC isolation: Peripheral blood samples are collected, and peripheral blood mononuclear cells are separated.
mRNA extraction and reverse transcription: mRNA is converted into cDNA.
Phage library generation: PCR is used to amplify the VHH gene regions.
Biopanning and library screening.
Positive clone sequencing and sequence analysis.
VHH expression and purification.
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Three types of VHH libraries are commonly used in antibody discovery:
Immunized VHH libraries, generated after camelid immunization with a specific antigen.
Naïve VHH libraries, generated from non-immunized camelids and used as a broad natural repertoire.
Synthetic or semi-synthetic VHH libraries, built in vitro using designed DNA sequences and engineered diversity.
Each approach answers a different discovery need. Immunized libraries are often preferred when the goal is to obtain high-affinity binders against a defined target. Naïve libraries can support faster screening when animal immunization is not desired. Synthetic libraries provide the greatest design control and can be adapted for specialized discovery strategies.
Immunized VHH libraries are generated by exposing a camelid to a target antigen so that the animal’s immune system produces antigen-specific heavy-chain antibodies. The antigen is the molecule the antibody should recognize. It may be a recombinant protein, peptide, cell-surface receptor, viral protein, toxin, or other biologically relevant target.
A typical immunized VHH library workflow includes several major steps.
First, a camelid is immunized with the antigen of interest. The immunization protocol may involve multiple injections over several weeks, depending on the antigen, adjuvant, animal model, and desired immune response. An adjuvant is a formulation component that helps stimulate the immune system.
Second, peripheral blood mononuclear cells, or PBMCs, are collected from the animal. PBMCs are immune cells found in blood, including B cells. B cells are the cells responsible for producing antibodies. After immunization, some B cells carry genetic information for VHHs that recognize the antigen.
Third, RNA is extracted from the PBMCs and converted into complementary DNA, or cDNA. RNA is the temporary genetic message produced from DNA, while cDNA is a stable DNA copy of that message. This step allows researchers to capture the VHH repertoire expressed by the animal’s B cells.
Fourth, the VHH sequences are amplified by polymerase chain reaction, or PCR. PCR is a molecular biology method used to copy specific DNA regions. Primers are designed to amplify VHH genes from the cDNA.
Finally, the amplified VHH sequences are cloned into a display system, most commonly phage display. Phage display is a screening method in which antibody fragments are displayed on the surface of bacteriophages, which are viruses that infect bacteria. Each phage carries the DNA sequence encoding the VHH it displays. This creates a physical link between binding function and genetic sequence, allowing researchers to identify both the binder and its DNA code.
During screening, the phage-displayed VHH library is incubated with the target antigen. Non-binding clones are washed away, while antigen-binding clones are retained, amplified, and subjected to additional selection rounds. This process is called biopanning.
The main advantage of immunized VHH libraries is that they capture an antigen-experienced immune repertoire. After immunization, B cells undergo affinity maturation in vivo. Affinity maturation is the natural process by which B cells accumulate mutations and are selected for stronger antigen binding. As a result, immunized libraries often provide a strong route to high-affinity, antigen-specific VHHs.
This can be particularly useful when the project requires potent binders to a defined target, functional blocking activity, or recognition of a biologically relevant conformation. Because the immune system has already selected B cells that respond to the antigen, the resulting library may be enriched for target-specific sequences.
Immunized libraries also preserve the natural single-domain structure of VHHs. Unlike single-chain variable fragments, or scFvs, VHHs do not require artificial pairing of heavy-chain and light-chain variable domains. This simplifies library construction and reduces issues related to incorrect chain pairing.
The main limitation is logistics. Immunized VHH library generation requires access to live camelids, animal handling infrastructure, immunization expertise, and appropriate ethical oversight. The process is also time-intensive. Because immune responses develop over multiple rounds of immunization, the full workflow can take weeks and, depending on the protocol, may extend to around two months or longer.
Some targets may also be challenging for immunization. Toxic antigens, poorly immunogenic antigens, highly conserved proteins, unstable membrane proteins, or antigens that are difficult to produce in native conformation may require specialized immunization and screening strategies.
For these reasons, immunized libraries are powerful, with contract research organizations (CROs) useful for bypassing these logistical bottlenecks. Specialized CROs like Biointron handle the animal housing, immunization, and library construction, making them a practical alternative to building and maintaining expensive, in-house camelid facilities.
Naïve VHH libraries are generated from non-immunized camelids. Instead of first exposing the animal to a specific antigen, researchers collect the natural VHH repertoire already present in healthy animals. This repertoire reflects baseline immune diversity rather than a target-specific immune response.
The construction process is conceptually similar to immunized library generation. PBMCs are collected, RNA is extracted, cDNA is synthesized, VHH genes are amplified by PCR, and the sequences are cloned into a display system such as phage display. However, because there is no antigen-specific immunization step, the library is intended to serve as a general-purpose source of VHH binders.
Naïve libraries can reduce project timelines because they avoid the immunization phase. This makes them useful when a rapid start is needed or when animal immunization is not feasible.
They can also support high-throughput screening across multiple targets. A well-constructed naïve library may be used repeatedly, allowing researchers to screen for general binders without generating a new immunized library for each antigen.
Naïve VHH libraries can be useful in early discovery campaigns where the immediate goal is to identify initial binding clones, map targetability, or generate starting points for subsequent engineering and affinity maturation.
Because naïve libraries are not enriched through antigen-specific immunization and in vivo affinity maturation, selected binders may have lower affinity than those obtained from immunized libraries. Affinity describes how strongly an antibody binds its target. High-affinity binders remain associated with the antigen more tightly, which can be important for therapeutic or diagnostic performance.
To compensate, naïve libraries typically require large diversity and robust screening strategies. Larger library size increases the probability that useful binders are present. However, a larger library also places greater demands on construction quality, display efficiency, screening conditions, sequencing, and clone triage.
Naïve libraries are therefore most useful when speed and broad applicability are prioritized, but they may require additional optimization to reach the desired affinity or functional performance.
Synthetic VHH libraries are built in vitro using designed DNA sequences rather than directly capturing a natural immune repertoire. Semi-synthetic libraries combine natural VHH framework features with engineered diversity in selected regions.
A framework is the structural scaffold of the VHH domain. It supports the overall fold and presents the antigen-binding loops. These antigen-binding loops are called complementarity-determining regions, or CDRs. CDRs are the most variable parts of an antibody and are usually the main contributors to antigen recognition.
In synthetic VHH libraries, researchers can use stable and well-characterized VHH frameworks, then introduce designed diversity into CDR regions. The goal is to create a library that maintains favorable developability properties such as solubility, stability, and expression while providing enough binding diversity for target screening.
Once a VHH library has been constructed, the next step is screening and selection. The goal is to move from a large pool of diverse VHH sequences to a smaller set of validated lead candidates.
Phage display remains one of the most widely used selection methods. During biopanning, the library is exposed to the immobilized or captured antigen. VHH-displaying phages that bind the antigen are retained, while weak or non-specific binders are removed by washing. Bound phages are then eluted and amplified for the next round. Multiple rounds of biopanning can enrich antigen-specific clones.
After selection, individual clones are screened. Enzyme-linked immunosorbent assay, or ELISA, is commonly used to test whether clones bind the target antigen. ELISA is a plate-based assay that produces a signal when binding occurs. Surface plasmon resonance, or SPR, and biolayer interferometry, or BLI, can provide more detailed binding kinetics. Binding kinetics describe how quickly a binder associates with and dissociates from its target.
Sequencing is then used to identify the DNA and amino acid sequences of selected VHH clones. Sequence analysis helps group related clones, identify unique families, remove duplicates, and prioritize candidates for expression and validation.
At this stage, researchers typically evaluate multiple properties, including target binding, specificity, affinity, expression yield, solubility, stability, and functional activity. For therapeutic development, additional properties such as humanization feasibility, immunogenicity risk, pharmacokinetics, and compatibility with downstream formats may also be considered.
Biointron supports VHH antibody discovery projects through integrated antibody discovery and engineering capabilities, including VHH screening, clone sequencing, recombinant expression, and binding validation. By aligning library strategy with target biology and downstream application, Biointron helps researchers move from antigen to candidate VHH sequences with a workflow designed for practical antibody discovery.
For teams developing therapeutic antibodies, diagnostics, research reagents, or engineered antibody formats, Biointron can support the key decision points in VHH library generation: whether to pursue an immunized, naïve, synthetic, or semi-synthetic approach; how to design screening conditions; and how to validate selected nanobody candidates.
As VHH technologies continue to expand into multispecific antibodies, cell therapies, imaging, intracellular tools, and difficult target classes, library design will remain a critical foundation. The right library can increase the likelihood of finding binders with the affinity, specificity, stability, and engineering potential needed for downstream success.
Hamers-Casterman, C., Atarhouch, T., Muyldermans, S., Robinson, G., Hamers, C., Songa, E. B., Bendahman, N., & Hamers, R. (1993). Naturally occurring antibodies devoid of light chains. Nature, 363(6428), 446–448. https://www.nature.com/articles/363446a0
Arbabi-Ghahroudi, M. (2022). Camelid Single-Domain Antibodies: Promises and Challenges as Lifesaving Treatments. International Journal of Molecular Sciences, 23(9). https://www.mdpi.com/1422-0067/23/9/5009
Muyldermans, S. (2021). A guide to: Generation and design of nanobodies. The Febs Journal, 288(7), 2084-2102. https://febs.onlinelibrary.wiley.com/doi/10.1111/febs.15515
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