Abstract

Embryos and newly hatched chicks appear unresponsive to mouse erythrocyte (MRBC) immunization as measured with the hemagglutinin (HA) and plaque-forming cell (PFC) assays. The competence for PFC formation first detected in the two day chick increased progressively with age. After grafting appropriate lymphoid cells into chick embryos or newly hatched chicks, PFC were observed consistently in the host spleens. Among embryo hosts grafted with blood leucocytes, spleen cells, thymus cells or bone marrow cells from antigen-primed donors, high PFC counts were obtained only in those that received either leucocytes or spleen cells. However, when the hosts were allowed to complete development and immunized with MRBC at hatching, moderate to high splenic PFC counts and HA production occurred in all groups, including those that received thymus or bone marrow cells. The data suggested that the immunologic immaturity may be due not to an inadequacy of the embryonic splenic microenvironment but to the deficiency of immunocompetent cells, particularly the thymus-derived helper cells.

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