sdab

Single domain antibody, also known as domain antibody, VHH, VNAR or sdAb, is a kind of antibody fragments consisting of a single monomeric variable antibody domain and lacking the light chain and CH domain of the heavy chain in conventional Fab region. The first single domain antibody was engineered from the VHH domain of heavy-chain antibody identified in camelids (e.g. dromedaries, camels, llamas, and alpacas). And the VNAR domain of cartilaginous fishes (e.g. shark) heavy-chain antibody (known as IgNAR, immunoglobulin new antigen receptor) is another qualified source for sdAb development. An alternative approach is to split the dimeric variable domains from normal IgG of humans or mice into monomers by camelizing a few critical residues.
https://www.creative-biolabs.com/nanobody-single-domain-antibody.html