domain antibody 

Single domain antibodies represent the smallest antibody that was proven of diagnostic and therapeutic usefulness. They are antibody fragments that engineered from single monomeric variable domains of either camelids’ heavy-chain antibody (VHH) or cartilaginous fishes’ IgNAR (VNAR). On the other hand, it is also possible to develop single domain antibodies from camelized human antibodies.
https://www.creative-biolabs.com/single-domain-antibody-library-service.html