Antibodies (Abs), also called immunoglobulin (Ig), are glycoproteins that play a central role in the humoral immunity. Conventional antibodies of all jawed vertebrates are heteromeric molecules which consist of two identical heavy (H) chains and two identical light (L) chains. This tetrameric structure had long been regarded as the only form of antibody until the discovery of a unusual class of antibody which is composed of two H chains and deprived from L chains (Hamers-Casterman et al., 1993).

The discovery of heavy chain-only antibodies (HCAbs) have many advantages over conventional antibodies that with higher thermal stability and higher solubility. To data, camelids are the only mammals that can produce heavy-chain antibodies. Due to their special properties, HCAbs have been employed as an exquisite research tool in structural, cell, and developmental biology. Furthermore, HCAbs have extend their application to the biomedicine.