Development of immune competence.

Present evidence indicates that the precursors of B- and T-cells can be found in the extra-embryonic tissues four days after implantation (day 5) and that by the following day (day 10) certain of the B-cell precursors have differentiated to the stage of the antigen-binding cell. Cells able to secrete antibody are not detected, however, until the later stages of pregnancy. B-cell differentiation has been shown to advance in a stepwise manner through several compartments, and the early stages of maturation are independent of thymic or T-cell regulation. The thymic rudiment appears by the 12th day of pregnancy and the reticuloepithelial tissue is quickly colonized by T-stem cells which migrate from the fetal liver. Within 4 days these cells respond to PHA, recognize and respond to allogeneic antigens, and begin to seed to the peripheral lymphoid tissues. During the latter stages of pregnancy and until about the sixth week after birth T-cell mediated suppressor activity predominates. T-cell killer function can be detected in the neonatal thymus shortly after birth, but this activity increases slowly in the peripheral lymphoid tissues. T-cell helper activity increases slowly after birth as suppressor activity declines. The adult levels of helper-suppressor function are approached about six to eight weeks after birth.