Lysis of foreign cells
is recognized by receptors in the placenta, permitting transport across to the fetus. Since IgG production is not optimal until the secondary response, women who have not been exposed to certain disease-causing agents that can infect and damage the fetus are warned to take extra precautions during pregnancy. For example, pregnant women are advised not to eat raw meat or become first-time cat owners; this is to avoid a primary infection by Toxoplasma gondii, a parasite that can be transmitted in raw meat and the feces of infected cats.
Maternal IgG antibodies not only protect the developing fetus against infections, but also the newborn because of the relatively long half-life ofthis class (figure 16.7). The protection provided by these maternal antibodies wanes after about 3 to 6
16.4 The Nature of Antibodies 401
months, but by this time the infant has begun to produce its own protective antibodies.
IgG is also present in colostrum, the first breast milk produced after birth. The intestinal tract of newborns is able to absorb this antibody.
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