Young CBA spleen cells respond initially to sheep red blood cells (SRBC) in vitro at 15 days of age but the response is less than 10% of the adult response. The magnitude of the response reaches levels comparable to those in the adult at 7 weeks of age. Spleen cells of 1-week-old mice can reconstitute the anti-SRBC response of lethally x-irradiated adult mice but cannot respond to SRBC when placed in diffusion chambers implanted in the peritoneal cavity of sub-lethally irradiated adult syngeneic recipients. In an attempt to determine the cellular basis of the “immunologic immaturity,” reciprocal recombinations of adult and young macrophage-rich adherent (A) cells and lymphocyte-rich non-adherent (NA) cells were made. It was found that young A cells are functional, but young NA cells are not. The implications of these findings in terms of the mechanism of maturation and the acquisition of immunocompetence by the young mouse are discussed. It is suggested that the cause of incompetence in the young spleen cells is the presence of pluripotent stem cells but not of committed lymphoid precursors. This may be due to the lack of functional lymphocyte-directed-inducing-microenvironments which would induce stem cells to differentiate into committed lymphoid precursors.

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