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Antibody formation by bone marrow cells in irradiated mice. I. Thymus-dependent and thymus-independent responses to sheep erythrocytes.

Bone marrow-thymus cooperation experiments were carried out in lethally irradiated mice with sheep red blood cells (SRBC) as the antigen and direct plaque-forming cells (PFC) as the end point. Various parameters were altered, with the following results: 
 
(1) Above 800 rad, the response by marrow cells alone, as well as the increase due to added thymus cells, was independent of irradiation dose. 
 
(2) The response of marrow cells was greatest at high SRBC concentrations, but the co-operative effect of thymus cells was most evident at lower SRBC levels, and completely absent at high levels. 
 
(3) Increasing the number of marrow cells, without thymus, gave increasing numbers of PFC, but the dose-response curve did not suggest cell synergism. 
 
(4) Thymectomy and antithymocyte serum treatment of host or donor did not prevent the response by marrow cells alone. It was concluded that this was a true IgM response by antibody-forming precursors from the marrow, unaided by thymus-derived cells.