Abstract
Summary
The cells involved in the initiation of the immune response in mice to sheep erythrocytes have been investigated by velocity sedimentation cell separation at unit gravity. The fractions obtained were assayed for immunocompetence by transplantation with sheep erythrocytes into lethally irradiated mice. Immunocompetent cells in mouse spleen were found to sediment as a single band with a sedimentation velocity of 3 mm/hr. Bone marrow and thymus are individually inactive in the transplantation assay but when mixed together produce a large immune response. It was shown that when bone marrow is sedimented, mixed with thymus, and assayed by transplantation, the major peak of activity is at 3 mm/hr. Thymus fractions, when mixed with bone marrow, also show a peak of activity at 3 mm/hr. From this it was concluded that the activity in spleen is probably due to two overlapping populations of cells at 3 mm/hr. It was shown that rosette-forming cells in both the bone marrow and spleen of unimmunized mice also have a sedimentation velocity of 3 mm/hr and it is inferred that these cells may be the direct progenitors of antibody-producing cells.
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