Maternal antibody IgG enters via Fc receptors and then becomes systemic with some leakage into the resp tract. This is why chickens need to be repeatedly vaccinated against NDV.

In an endemic situation maternal antibody , IgG, will delay virus replication in animals. The animals develop subclinical disease when a few weeks old instead of clinical disease when a few days old e.g. Porcine respiratory and reproductive syndrome (PRRS).