The avian embryo and its accessibility in the egg offer significant advantages for the analysis of immune system development. The separate developmental pathways ofthymus-dependent T cells and bursa-dependent B cells (1-4) and their derivation from hemopoietic stem cells (SC)' (5-7) were first disclosed in studies of chick development. Chick-quail chimeras were used to show that blood-borne stem cells periodically migrate into the thymus in response to chemoattractants produced by thymic epithelial cells (8, 9). The first wave of stem cell influx into the thymus begins on the fifth day of embryonic development (E5) in the quail and on E6.5 in the chick, and this influx persists for 1-2 d. At least two additional waves of stem cells enter the embryonic thymus after nonreceptive intervals of ti4 d (9, 10).The development of mAbs with specificity for the chicken TCRs and associated cell surface molecules (CD3, CD4, CD8) has allowed detailed study of the T cell progeny of hemopoietic stem cells (11-18). Sequential development of three sublines of daughter T cells has been defined using these antibodies. The first consists of T cells expressing the avian TCR-y/b homologue in association with the avian CD3 protein complex (13). These TCR1/CD3-bearing cells appear first in the E12 thymus .T cells expressing the avian a/a receptor complex, TCR2/CD3, appear on E15 (14-16).As in mammals, the immature TCR2 cells express both CD4 and CD8 molecules, whereas the TCR1 thymocytes express neither. A third T cell sublineage has recently been identified as cells expressing a different receptor complex (17, 18). These TCR3/CD3+ cells begin to appear in the thymus on E18.The present experiments, using chick-quail chimeras constructed by embryonic thymus engraftment, were primarily designed to examine lineage relationships among the different waves of stem cells and their intrathymic progeny. Specifically, we sought

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A prospective study on the British and Japanese BCG vaccines in newborn infants was carried out in which 317 and 285 infants were randomly allocated and vaccinated with the British and Japanese BCG vaccines, respectively. Four follow-up examinations were carried out with an average of 98% of the study cohort attending all sessions. About half the infants did not produce any visible response at the end of the first week. All, by the end of the third month, had characteristic BCG scars with an average diameter of 4.6 mm. A Mantoux test was carried out at six months. A mean skin induration of 7.2 mm (SD 5.3 mm) was recorded. Significantly higher proportions of infants given Japanese BCG were found to be tuberculin convertors (74.7%) when compared to those given British BCG (51.4%). Breast-feeding practices and the mothers' tuberculin status did not influence markedly their infants response to tuberculin.