B cells diferentiate from pluripotent hemopoietic stem cells that migrate to the liver (Melchers, 2015; Muzzio, 2013). Despite this, in the absence of a direct antigenic stimulus such as infection, fetal plasma immunoglobulins consist almost totally of transferred maternal immunoglobulin G (IgG). hus, antibodies in the newborn are most often relective of maternal immunological experiences (American College of Obstetricians and Gynecologists, 2017). he interaction between maternal and fetal T cells is described in detail in Chapter 5 (p. 95).
Maternal IgG transport to the fetus begins at approximately 16 weeks and increases thereafter. he bulk of IgG is acquired during the last 4 weeks of pregnancy (Gitlin, 1971). Accordingly, preterm neonates are poorly endowed with protective maternal antibodies. Newborns begin to slowly produce IgG, and adult values are not attained until age 3 years. In certain situations, the transfer of IgG antibodies from mother to fetus can be harmful rather than protective to the fetus. The classic example is hemolytic disease of the fetus and newborn resulting from Rh-antigen alloimmunization (Chap. 15, p. 301).
In the adult, production of immunoglobulin M (IgM) in response to an antigenic stimulus is superseded in a week or so predominantly by IgG production. In contrast, very little
IgM is produced by normal fetuses. With infection, the IgM response is dominant in the fetus and remains so for weeks to months in the newborn. And, because IgM is not trans ported from the mother, any IgM in the fetus or newborn is that which it produced. hus, specific IgM levels in umbilical cord blood may be useful in fetal infection diagnosis. Accord ing to the American College of Obstetricians and Gynecologists (2017), elevated levels of IgM are usually found in newborns with congenital infection such as rubella, cytomegalovirus infection, or toxoplasmosis. In infants, adult levels of IgM are normally attained by age 9 months.
mucosal protection against enteric infections. This may explain the small amount of fetal secretory IgA found in amnionic luid (Quan, 1999).
he immune system develops early, and B lymphocytes appear in fetal liver by 9 weeks and in blood and spleen by 12 weeks. T lymphocytes begin to leave the thymus at approximately 14 weeks. Despite this, the newborn responds poorly to immunization, and especially poorly to bacterial capsular polysaccharides. his immature response may stem from a deicient response of newborn B cells to polyclonal activators or from a lack of T cells that proliferate in response to specific stimuli (Hayward, 1983). In the newborn, monocytes are able to process and present antigen when tested with maternal antigen-speciic T cells. DNA methylation patterns are developmentally regulated during monocyte-macrophage diferentiation and contribute to the antiinlammatory phenotype in macro phages (Kim, 2012).
The origin of most muscles and bones is mesodermal. he skeleton arises from condensed mesenchyme-embryonic connective tissue-which eventually forms hyaline cartilage models of the bones. By the end of the embryonic period, ossification centers have developed, and bones harden by endochondral ossification. The limb buds appear by the fourth week. Most skeletal muscle derives from myogenic precursor cells in the somites.
Because of the small amount of yolk in the human ovum, growth of the embryofetus is dependent on maternal nutrients during the irst 2 months. During the first few days after implantation, blastocyst nutrition comes from the interstitial fluid of the endometrium and the surrounding maternal tissue.