IgG vs IgM in the fetus. Berkley Review Immunology

[manohman]

the question reads:

"Treponema palladium, an organism that causes syphilis, can cross the placental membrane and enter into fetal circulation. Which of the following immunoglobins would be expected to increase in concentration in the blood serum of the fetus?"

The answer is IgM, and the answer key says its because IgM is the first antibody produced by plasma cells in an immune response.

That makes sense, but given that IgG is the only antibody that can cross the placental membrane and in the greatest concentration in rhe fetus normally, why wouldnt it be igg?

Would IgG not increase as well? And if not, then what does IgG do?

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[techfan]

Well the way I understand it, IgM is less specialized and is decavalent so naive B cells throw these "native" IgM's out first since they can bind to almost any epitope. IgGs are produced when the Antigen presenting cell (macrophages etc.) takes the epitope and has the B cell make a specific antibody to it which is usually IgG. Yes, IgGs cross the placental membrane, but since they would be coming in a constant stream from the mother the concentration wouldn't rise whereas the fetus would start making more IgM.