F, and HLA-G. Recognition of these HLA class Ib proteins by natural killer cells residing within the decidua inhibits their activ ity and promotes immune quiescence (Djurisic, 2014).
Another immune adaptation that promotes tolerances stems from important changes in CD4 T lymphocyte subpopulations in pregnancy. First, Th1-mediated immunity shifts to h2-mediated immunity. Indeed, an important antiinlammatory component of pregnancy involves suppression ofT-helper (Th) 1 and T-cytotoxic (Tc) 1 cells, which lower secretion of interleukin-2 (IL-2), interferon-a, and tumor necrosis factor (TNF). Moreover, suppressed Th1 response is thought to be a requisite for pregnancy continuation. It also may explain pregnancy-related remission of some autoimmune disorders such as rheumatoid arthritis, multiple sclerosis, and Hashimoto thyroiditis-which are cell-mediated immune diseases stimulated by h1 cytokines (Kumru, 2005). With suppression of Th1 cells, there is upregulation of Th2 cells to increase secretion of IL-4, IL-10, and IL-13 (Michimata, 2003). hese h2 cytokines promote humoral, or antibody-based, immunity. hus, autoimmune diseases mediated mainly by autoantibodies, such as systemic lupus erythematosus, may lare if the disease is already active in early pregnancy. But, the transition to an antibody-mediated immunity is an important defense during pregnancy and early puerperium. In cervical mucus, peak levels of immunoglobulins A and G (IgA and IgG) are significantly higher during pregnancy, and the immunoglobulin-rich cervical mucus plug creates a barrier to ascending infection (Hansen, 2014; Wang, 2014). Similarly, IgG is transferred to the developing fetus in the third trimester as a form of passive immunity, ostensibly in anticipation of birth. Further, immunoglobulins secreted into breast milk during lactation augment neonatal defenses against infection.
Other subpopulations of CD4 T lymphocytes serve mucosal and barrier immunity. These specific CD4-positive cells are known as h17 cells and Treg cells. h17 cells are proinflammatory and express the cytokine IL-17 and the retinoic acid receptor-related orphan receptors (RORs). Treg cells express the transcription factor forkhead box protein-3 (FOXP3) and confer tolerizing activity. here is a shift toward Treg CD4 cells in the first trimester, which peaks during the second trimester and falls toward delivery (Figueiredo, 2016). his shift may promote tolerance at the maternal-fetal interface (La Rocca, 2014). In particular, failure of these CD4 T lymphocyte subpopulation alterations may be related to preeclampsia development (Vargas-Rojas, 2016).
nonpregnant values, and upper values approach 15,000/jLL (Appendix, p. 1255). During labor and the early puerpe rium, values may become markedly elevated, attaining levels of 25,000/LL or greater. The cause is unknown, but the same response occurs during and after strenuous exercise. The leuko cytosis possibly represents the reappearance of leukocytes previ ously shunted out of active circulation.
The distribution of lymphocyte cell types is also altered during pregnancy. Speciically, B lymphocytes numbers are unchanged, but the absolute numbers of T lymphocytes rise and create a relative increase. Concurrently, the ratio of CD4 to
CD8 T lymphocytes does not change (Kuhnert, 1998).