The duck IgY molecule, consisting of two heavy (H) and two light (L) chains, has been categorized as the 7.8S full-length IgY form and the 5.7S truncated IgY(-Fc) form. Studies have shown that the IgY(-Fc) molecule possesses the H chain Cu1 and Cu2 constant domains, but is lacking the Cu3 and Cu4 constant region terminal domains, all four of which are present in the full-length IgY molecule. Thus the structure of the IgY(-Fc) molecule resembles that of a F(ab’)2 fragment of IgY.