Humans and most other mammals produce antibodies consisting of two heavy chains, each linked to a light chain. Both heavy and light chain contribute to the variable region, and thus the antigen specificity of the antibody.
In contrast, cartilaginous fish and camelids have been found to produce heavy-chain-only antibodies, and thanks to their small size and ease of synthesis, camelid antibodies (nanobodies) are predicted to revolutionize antibody therapeutics.
This makes me wonder: why is this not possible with human (genetically engineered) antibodies? Apart from the fact that removing the light chain would alter antigen specificity, what hinders a heavy-chain-only human antibody from being functional? What is so fundamentally different between the camelids heavy chain (and their variable region) and the human equivalent?