Immune responsiveness of neonatal chicks and immunocyte precursors from unprimed juvenile chicken donors.

The immune responsiveness of immunocyte precursors, obtained from peripheral blood of 4- to 6-wk-old unprimed chickens and assayed with the cell transfer model, was compared with that of neonatal chicks. Little if any splenic plaque-forming cell (PFC) production was observed in the spleens of embryonic cell transfer hosts following a single antigen exposure to weak or moderate dosages of mammalian erythrocytes. However, modest to high PFC formation occurred consistently in the hosts following a second exposure to the same antigen when given 3 days later. Although a single immunization failed to elicit PFC formation in embryo hosts, a single injection of a wide range of antigen dosages into 8-day-old chicks elicited PFC production consistently. When the double immunization protocol was used with 2- to 9-day-old neonatal chicks, the level of immune responses were indistinguishable from those elicited with a single antigen injection. The difference in the immune kinetics observed in embryo hosts may be related to the embryonic microenvironment, which is permissive, but may not be as immunosupportive as that of neonates.