A conventional antibody is a four-chain assembly: two heavy chains and two light chains, with the binding site formed where they meet. In the early 1990s researchers examining camel blood found a second antibody type circulating alongside the usual one, built from heavy chains only. Llamas, alpacas and the other camelids share it.
Because the binding site sits in a single domain, that domain can be expressed on its own. The resulting fragment, about a tenth the mass of a whole antibody, is often called a nanobody. It folds robustly, tolerates heat and can reach clefts on a target protein that a bulkier antibody cannot enter. Llamas are routinely immunised to generate them for laboratory reagents and imaging.
The approach reached the clinic with caplacizumab, a therapy for a rare clotting disorder derived from llama-raised single-domain antibodies. Why camelids evolved heavy-chain-only antibodies at all, and what advantage they confer in the animal itself, is still unexplained.