IgY versus IgG

what are the differences?

Avian IgY is genetically and structurally different from its mammalian counterpart IgG. 
As igG, IgY has 2 heavy chains and 2 light chains shaped in the characteristic Y-shape with an antigen valency of 2. Functionally, they play similar biological roles. IgG and IgY are both major immunoglobulins providing defense against infectious agents and appearing in the bloodstream at high concentrations following exposure to an antigen. 

IgY has a higher molecular weight due to an extra heavy chain constant domain. IgY also has a unique glycosylation pattern. However, there is little or no immunological cross-reactivity between IgY and mammalian IgG. In chickens IgY is actively transported to the egg yolk where the IgY concentration is ultimately higher than in blood.