A Practical Approach, Second Edition=Ronald D. Ho.pdf

POSTNATAL DEVELOPMENTAL MILESTONES 1089lower rate of Ig synthesis, with IgA and IgM more affected than IgG, and have approximately 10-fold lower numbers of plasma cells.HumansAt the time of human birth, the proportion of lymphocytes represented by T-cells is lowest andincreases over time (Semenzato et al., 1980; Series et al., 1991; Berry et al., 1992; Erkellar-Yukselet al., 1992; Hannet et al., 1992; Plebani et al., 1993; Hulstaert et al., 1994). Specifically, absolutenumbers of both CD4 + and CD8 + subsets increase, while the CD4/CD8 ratio has been variablyreported to differ, or not, with time after birth (Foa et al., 1984; Hannet et al., 1992; Plebani et al.,1993). Usage of TcR Vβ subsets likewise has been variably reported to differ, or not, betweennewborn and adult αβ-T-cells (Foa et al., 1984; Hayward and Cosyns, 1994). The proportion of Tcells expressing the γδ-TCR is high during fetal life, decreases at term, but remains higher innewborns than later in life (Peakman et al., 1992). Some reports have described circulating ‘doublepositive(CD4 + CD8 + ) immature’ T-cells; however generally positive and negative selection areprobably complete by birth (Foa et al., 1984; Griffiths-Chu et al., 1984; Solinger, 1985; Reason etal., 1990; Hayward and Cosyns, 1994). Nonetheless, T-cells do have phenotypic and functionalfeatures of ‘naïve’ cells. In particular, markers of ‘immature’ or antigenically ‘naïve’ cells such asCD45RO - /RA + on CD4 + cells are much higher than in adults (Clement et al., 1990; Denny et al.,1992; Erkellar et al., 1992; Hannet et al., 1992; Hulstaert et al., 1994; Igegbu et al., 1994; Jenningset al., 1994). Expression of some surface molecules (e.g., integrins, adhesion molecules) is reportedto be low at birth and increases post-natally, while expression of others is high and decreases(Clement et al., 1990; Hannet et al., 1992; Hayward and Cosyns, 1994).MiceFollowing birth, there is an immediate disappearance of hematopoietic cells from the liver as thatorgan assumes postnatal function. In the postnatal mouse, leukocytes are produced in the bonemarrow and, except for T lymphocytes, complete their maturation in that tissue. It is known thatsplenic hematopoiesis persists for several weeks after birth in rodents (Marshall-Clarke et al., 2000).Spear et al. (1973) observed an increase in B-cells and a decrease in the T-cell to B-cell ratio inmice between 2 and 3 weeks of age, which coincided, with the onset of antigen responsiveness intheir studies. As discussed below, similar results were presented in 21-day old rats by Ladics et al.(2000).One of the clearest indications that the immune system continues to develop in mice is the factthat perinatal (