Even after intensive study, many of the maternal immunological adaptations to pregnancy are not well elucidated. It is known that pregnancy is associated with an increase in the CD4+ T cells that secrete h2-type cytokines-for example interleukins (Fragiadakis, 2016). Th1-type cytokine production-for example, interferon gamma and interleukin 2-appears to be somewhat suppressed, leading to a h2 bias in pregnancy. This bias afects the ability to rapidly eliminate certain intracellular pathogens during pregnancy, although the clinical implications of this suppression are unknown (Kourtis, 2014; Svensson-Arvelund, 2014). Importantly, the h2 humoral immune response remains intact. It also appears that human leukocyte antigen (HA)-C expressed by extravillous trophoblasts elicits responses from decidual natural killer (dNK) and decidual CD8+ T cells (Crespo, 2017).
In describing infections, horizontal transmission is the spread of an infectious agent from one individual to another. Vertical transmission refers to passage from the mother to her fetus of an infectious agent through the placenta, during labor or delivery, or by breastfeeding. Thus, preterm rupture of membranes, prolonged labor, and obstetrical manipulations may enhance the risk of neonatal infection (Centers for Disease Control and Prevention, 2010). Table 64-1 details speciic infections by mode and timing of acquisition. A inal term, the seconday attack rate, is the probability that infection develops in a susceptible individual following known contact with an infectious person.
The active immunological capacity of the fetus and neonate is compromised compared with that of older children and adults. That said, fetal cell-mediated and humoral immunity begin to develop by 9 to 15 weeks' gestation (Warner, 2010). The primary fetal response to infection is immunoglobulin M (Ig.1). Passive immunity is provided by IgG transferred across the placenta. By 16 weeks, this transfer begins to rise rapidly, and by 26 weeks, fetal concentrations are equivalent to those of the mother. After birth, breastfeeding is protective against some infections, although this protection begins to decline at
TABLE 4-1 . Specific Causes of Some Fetal and Neonatal Infections
Viruses: varicella-zoster, coxsackie, human parvovirus B19, rubella, CMV, HIV, Zika
Bacteria: Listeria, syphilis, Borrelia
Protozoa: toxoplasmosis, malaria group B streptococcus, coliforms Viruses: HIV
Bacteria: gonorrhea, chlamydia, group B streptococcus, tuberculosis, mycoplasmas Viruses: HSV, HPV, HIV, hepatitis B, hepatitis C, Zika External contamination
Bacteria: staphylococcus, coliforms
Viruses: HSV, varicella zoster
Human transmission: staphylococcus, HSV
Respirators and catheters: staphylococcus, coliforms
CMV = cytomegalovirus; HIV = human immunodeficiency virus; HPV = human papillomavirus; HSV = herpes simplex virus.
2 months of age. Current World Health Organization (2013) recommendations are to exclusively breastfeed for the first 6 months of life with partial breastfeeding until 2 years of age.