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Immunoglobulins (IgA, IgA1, IgA2, and IgG) are measurable in AF samples between 11 and 40 weeks of gestation. Whereas IgG, IgD, and IgA levels increase from 11 to 25 weeks and then decrease until term, IgM levels tend to remain constant until 35 weeks and then increase until term.³⁸⁰ Davis et al.³⁸¹ found similar IgG levels in AF from the second trimester. Their study, however, dealt especially with antibodies to herpes simplex virus (HSV) type 1, which was found in 78 percent of AF samples tested. Antibodies to cytomegalovirus (CMV) were found in 84 percent of the same AF samples tested. No viruses, bacteria, mycoplasma, or chlamydiae were isolated from the samples. Isolation of CMV has been successful in AF from two fetuses showing severe growth restriction and classic CMV infection³⁸² (see Chapter 34).

The immunologic activity of AF remains poorly understood and much remains to be learned about mechanisms involved in neonatal immune disease in babies born to mothers affected with systemic lupus erythematosus, idiopathic thrombocytopenic purpura, Graves disease, and myasthenia gravis. Auger et al.³⁸³ studied the antibody response to Candida albicans during the second trimester. Specific IgG was detected in 94.7 percent of the samples and specific IgA in 98 percent. There was a predominance of IgA activity in the AF. There was no correlation between the IgG and IgA titers, suggesting a fetal origin for IgA, which would offer a functional advantage over maternally transmitted IgG. Immunoglobulin C declines as pregnancy progresses toward term.³⁸⁴