Newly hatched birds emerge from the sterile environment of the egg. Serum immunoglobulins are actively transported from the hen’s serum to the yolk while the egg is still in the ovary. During egg production about 30% of the hen’s immunoglobulin (IgY) will transfer from her plasma to the yolk. IgY in the fluid phase of egg yolk is therefore found at levels equal to or greater than those in hen serum. As the fertilized ovum passes down the oviduct, IgM and IgA from oviduct secretions are acquired with the albumin. As the chick embryo develops in ovo, it absorbs the yolk IgY, which then appears in its circulation. At the same time, the IgM and IgA from the albumin diffuse into the amniotic fluid and are swallowed by the embryo. Thus when a chick hatches, it possesses IgY in its serum, and IgM and IgA in its intestine. The newly hatched chick does not absorb all its yolk sac antibodies until about 24 hours after hatching. These maternal antibodies effectively prevent successful vaccination until they disappear between 10 and 20 days after hatching. Newly hatched chicks begin to make their own IgA at day three in the bursa, and day seven in the gut and lung. Interestingly, maternal IgA persists for at least seven days because it appears to be retained by the intestinal mucus. The presence of maternal antibodies may neutralize live vaccine strains and day of age vaccination is not therefore ideal. That is why in ovo vaccination is employed.