A conventional antibody is a Y-shaped assembly of two heavy and two light chains, with the antigen-binding site formed where they meet. Camelids, including llamas, alpacas and camels, additionally make heavy-chain-only antibodies whose binding site sits in a single domain. The discovery was made in Brussels in 1993 during a student practical on dromedary serum.
That single domain, isolated, is roughly a tenth the mass of a full antibody. It is stable at high temperature, folds reliably, can be produced in bacteria and penetrates dense tissue. Its elongated binding loop reaches into enzyme active sites and other clefts that a flat conventional binding surface cannot enter. Sharks independently evolved a comparable single-domain antibody.
The first nanobody-derived medicine, caplacizumab, was approved in Europe in 2018 and in the United States in 2019 for a rare clotting disorder. Nanobodies raised in llamas were also investigated intensively against SARS-CoV-2. Llamas are used for immunisation in practice because they are more tractable to handle than camels and produce a broad response.