The cellular component includes all the cells that react with specificity to antigens, except those associated with antibody production. The cellular system was identified when it was shown that chickens with damaged BF could still respond to and eliminate many disease organisms. A chicken may become immune to a disease organism by producing antibodies itself or by obtaining antibodies from another animal. 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.