Thymus dependency of antigen-binding cells in the bone marrow and spleen of chicks.

The bone marrow and spleen of normal unimmunized chicks contain lymphoid cells which form rosettes with quail red blood cells (QRBC), but not with sheep red blood cells. The number of these antigen binding cells (QRBC-ABC) were restored to their original level 2-3 weeks after whole body X-irradiation following a temporary but drastic decrease in the number of QRBC-ABC. This recovery is not observed in chicks which have been thymectomized or thymectomized and bursectomized. However, QRBC-ABC numbers were restored in bursectomized chicks comparable with those levels in control chicks. Anti-thymic serum effectively blocked rosette formation and it seems that QRBC-ABC are a thymus-dependent population. Possibly, the thymus exerts an influence on the genesis of QRBC-ABC through a humoral mechanism.