When the chicken produces its own antibodies following exposure to a foreign material, the process is called active immunity. This occurs after the bird is exposed to a vaccine or a disease. Active immunity is harmed by anything that damages the cellular or humoral immune systems. When the chick receives pre-made antibodies from the hen through the egg, this is termed passive immunity. These antibodies are not produced by the chick.
Maternal antibodies are present in the yolk, albumin, and the fluids of the egg.
If the hen has a high antibody titer level to a disease, the chick should also be immune for several weeks. However, since the immune system of the chick is not stimulated, there will be no antibodies produced by the chick and no memory cells. The flock manager must be aware of the maternal antibody levels in the chicks to schedule vaccinations. If chickens are vaccinated when maternal antibody titer levels are elevated, the vaccine may be buffered excessively, resulting in reduced response. Conversely, if vaccinations are delayed and maternal titer levels are low, a severe vaccine reaction may result.