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

POSTNATAL DEVELOPMENTAL MILESTONES 1091PrimatesA study by Neubert et al. (1996) compared a number of phenotypic markers in peripheral bloodlymphocytes between adults and newborns in both humans and primates (marmosets). Whileperipheral blood was used for adult humans, adult marmosets and newborn marmosets, umbilicalcord blood was used as a surrogate for newborn humans. Their results indicated no significantdifferences between newborns and adults in either humans or primates and only trends will bediscussed.* % lymphocytes: humans: umbilical = adultmarmosets: newborn ≤ adult* CD4 + (helper): humans: umbilical = adultmarmosets: newborn = adult* CD4 + /CD29 + (memory): humans: umbilical < adultmarmosets: newborn < adult* CD4 + /CD45RA (naïve): humans: umbilical > adultmarmosets: newborn >> adult* CD8 + (suppressor): humans: umbilical ≤ adultmarmosets: newborn ≥ adult* CD 56 + (NK cells): humans: umbilical = adultmarmosets: newborn = adult* CD 20+ (B-cells): humans: umbilical = adultarmosets: newborn = adultThe biggest differences between newborns and adults observed in this study were for memoryT-cells (e.g., CD4 + /CD29 + ), which were lower in newborns, and for naïve T-cells (CD4 + /CD45RA),which were higher in newborns. Both of these observations are consistent with, and do not detractfrom the interpretation that the immune systems of humans and primates are devoid of any antigenicexposure, but are effectively mature at birth. Neubert et al. (2002) emphasized that the immunesystem of newborns (e.g., marmosets and humans) is rather ‘immature’; that these deficiencies arenot so much an indication of a lack of important components of the immune system, but rather arethe result of little intrauterine contact with environmental antigens. The biggest difference betweenhumans and marmosets was in the number of white blood cells (e.g., measured either as total WBCsor as % lymphocytes) being considerably higher in the primates.Acquisition of Functional ImmunocompetenceAs noted above, the onset of functional immunocompetence varies across species and is strikinglydifferent between rodents and humans. Exposure to a specific antigen during the perinatal periodresults in a rapidly expanding accumulation of lymphocyte specificities in the pool of memory cellsin secondary lymphoid tissues. As thymic function wanes and thymocytes are no longer producedin that tissue, it is this pool of memory B- and T-cells that maintains immunocompetence for thelife of the individual.HumansThymocytes derived from human fetal tissue of less than 11 weeks gestation show no demonstrableresponse to mitogen stimulation (Kay et al., 1970; Sites et al., 1974; Royo et al., 1987). Functionalcapability of human fetal T-lymphocytes begins to develop between the end of the first trimesterand the end of the second trimester (~14-26 weeks). At 12-14 weeks, fetal thymocytes respond toPHA only, while by 13-14 weeks (after thymic colonization with stem cells), fetal thymocytesrespond to most mitogens (Kay et al., 1970; Sites et al., 1974; Royo et al., 1987). Mitogen-responsive© 2006 by Taylor & Francis Group, LLC