Alpaca heavy chain antibody (HcAb) is a naturally occurring special antibody composed of only two heavy chains, including only one heavy chain variable domain (VHH in Antibody) and two conventional CH2 and CH3 domains . Alpaca heavy chain antibodies specifically bind to antigens through a variable domain (VHH) on the heavy chain, which can exist stably in vitro alone, named nanobody or VHH. The nanobody crystal has a diameter of 2.5nm and a length of about 4nm. The molecular weight of the nanobody is only 1/10 (about 15KD) of the conventional antibody, but it has complete antigen recognition ability.