Chicken IgY is the major circulating antibody found in chickens and is the avian counterpart to mammalian IgG. IgY is equivalent to IgG in most experimental applications, including Western Blot, ELISA, Immunohistochemistry, Immunocytochemistry, and function blocking experiments.

IgY has the same general structure as mammalian IgG, with two heavy chains ("nu" chains at ~67-70 kDa) and two light chains (at 22-30 kDa) (see figure).

The molecular weight of the whole IgY molecule is about 180 kDa, but it often runs as a smear on gels due to the presence of about 3% carbohydrate. Heavy chains of IgY are composed of four constant domains and one variable domain, which contains the antigen-binding site. Unlike the gamma-chains of IgG, however, the heavy chains of IgY lack Fc domains, which means that IgY neither fixes, complements, nor binds protein A or protein G. Chickens also produce an alternatively-spliced form of the heavy chain in which the Cnu3 and Cnu4 domains are deleted. In some individual hens, this F(ab)2-like heavy chain can be seen running at 40 kDa in SDS-(10%) polyacrylamide gels.