What type of immunity involves the transfer of antibodies from a hen to her chick through the egg yolk?

The correct answer is passive immunity, which refers to the transfer of pre-formed antibodies from one individual to another. In the context of poultry, when a hen lays eggs, she provides her chicks with immunity by transferring antibodies through the egg yolk. This is particularly important for the chicks as it offers them immediate protection against pathogens immediately after hatching, since their immune systems are not fully developed at that stage.

Passive immunity is temporary; the antibodies received from the hen will eventually decline as the chicks grow, emphasizing the importance of the hen's health and antibody levels prior to laying eggs. In contrast, active immunity is developed through the exposure to antigens, either through infection or vaccination, resulting in the hen’s immune system producing its own antibodies over time. Cell-mediated immunity involves T-cells and is a different mechanism of immune response not directly related to the transfer of antibodies. Innate immunity refers to the general, non-specific defenses the body has against pathogens that do not rely on prior exposure or specific antibodies.