AP BIOLOGY

Detailed explanation-1: -IgG is the only antibody class that significantly crosses the human placenta.

Detailed explanation-2: -The transport of IgG antibodies across the placenta is known to occur especially in the third trimester of pregnancy, and as more time passes, more antibodies cross. Scientists also know that an infection with a novel virus can take time to elicit a significant antibody response.

Detailed explanation-3: -Immunoglobulin M (IgM) does not cross the placenta; it is a result of fetal immunologic response to pathogens, an ability that is acquired early in the first trimester of pregnancy. IgG in the fetal blood at birth is a result of maternal immunoglobulin transferred across the placenta.

Detailed explanation-4: -immunoglobulins-IgA and IgM are excluded from crossing the placenta. Thus IgM antibodies in the newborn indicate a response to intrauterine infection. IgG is selectively transferred across the placenta.

Detailed explanation-5: -the placenta is deficient in transporter receptors to IgG2.