Spleens from newborn mice less than 6‐7 days of age contain a “naturally occurring” suppressor cell which possesses the capacity to inhibit not only expression of alloreactivity in the newborn spleen per se, but also T‐dependent and T‐independent immune responses of third‐party adult spleen cells. Similar suppressor activity is not detectable in the newborn thymus. The effector cell of this suppressor activity is not a classical T cell: suppression is resistant to treatment with anti‐T cell reagents, is absent from purified spleen T cell populations, is present in the spleens of T‐deficient (nu/nu) mice, is Helix pomatia A hemagglutinin‐nonbinding. Furthermore, this cell appears to be distinct from classical B cells, natural killer (NK) cells and macrophages. Instead, this naturally occurring, newborn spleen‐associated suppressor cell has similar characteristics to the non‐T suppressor cell detectable during old‐age immunological senescence, described by Singhal et al. (Fed. Proc. 1978. 37: 1245). When this suppressor cell is removed from newborn spleen cell populations, the resulting T cells exhibit strong alloreactivity. Thus, these data indicate, first, that the nonresponsive state of the newborn spleen is not due to a lack of functional T cells or macrophages, and second, that the suppressor activity in the newborn spleen is not effected by a thymus‐ dependent cell as is currently hypothesized.

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