VHH

VHH, or the variable domain of heavy-chain antibodies, denotes the antigen-binding fragment of heavy-chain-only antibodies found in camelids. In camelids, certain antibodies naturally lack light chains and CH1 domains, and their single variable domain, VHH, can function as a complete antigen-binding unit. VHHs are also referred to as nanobodies. They are small, typically 13–15 kilodaltons, around 2.5 nanometers in diameter, and bind with high specificity and affinity to diverse antigens, including enzyme active sites and recessed epitopes.

Structurally, VHHs retain a conventional VH-like fold but possess distinctive framework-region residues that enhance solubility and stability and a particularly long complementarity-determining region 3 (CDR3) that often contacts the antigen. Because of their small size and stability, VHHs can be produced in microbial systems, and display libraries enable selection against challenging targets, while they tolerate a wide range of pH and temperatures.

In practice, VHHs are used as research reagents, diagnostic tools, and therapeutic agents. They can be engineered into multivalent or bispecific formats or fused to Fc regions to extend plasma half-life. Therapeutic nanobodies include Caplacizumab (Cablivi), a humanized VHH that targets von Willebrand factor to treat acquired thrombotic microangiopathy. In imaging and therapeutics, VHHs offer advantages such as tissue penetration and potential penetration of the blood–brain barrier when appropriately engineered. Potential drawbacks include immunogenicity concerns and the need for humanization or careful design for clinical use.