Kinetic analysis of the enhanced immune responsiveness of baby chicks grafted with lymphoid cells from juvenile donors.

The immunologic immaturity of embryos and newly hatched chicks to MRBC immunization has provided us with a model to investigate the distribution of immunocytes and their precursors in several tissues of juvenile chickens. Antigen-responsive cells (ARC) were rare in the peripheral blood, spleen, bone marrow, thymus and bursa of Fabricius of unimmunized young birds. Following MRBC immunization, they were abundant in the blood and spleen, low in frequency in bone marrow and absent in the thymus and bursa. The recipients of bone marrow cells, thymocytes, or bursacytes, when immunized at hatching, exhibited accelerated humoral immune responses. The enhancement property of the grafted cells was increased when MRBC was included in the cell suspension at the time of grafting. The precocious responsiveness of the grafted chicks is attributed to the cooperation of donor and host immunocytes. The enhancement effect, moreover, was less consistently expressed in allogeneic hosts. This capacity was aquired by thymic cells within a day after immunization of the donor and persisted well over a week. The role of antigen in driving the specific differentiation of T and Bu cells is briefly discussed.