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Thread: why does only IgG cross the placenta?

  1. #1 why does only IgG cross the placenta? 
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    we all know dat IgG crosses d placenta but can u please explain me d reason for that?


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  3. #2  
    Forum Ph.D.
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    I think its because it exists in the plasma as a dimer. IgM is a pentamer and so too large, IgE shouldnt normally be in the blood in large amounts and IgD cell membrane bound/dont move around in circulation, and IgA is only found in mucous tracts.


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  4. #3  
    Universal Mind John Galt's Avatar
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    Because if it didn't the fetus would be immunilogically unprotected and survival rates would be low to zero and none of us (including aardvarks) would be here. So, it is an artifact of natural selection.
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  5. #4  
    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    IgG exist as a monomer in the plasma, IgM is a pentamer as Robbie said. IgA exist as a dimer thus can not cross the placenta, and is mostly localized to mucousal layers. IgD is bound to the surface of B cells, which do not cross the placenta. IgE is in very very low levels in the plasma and mostly bound by eosinophils.

    So the only ones small enough to pass the placenta are IgG and IgE, IgE is not found in large enough amounts in the plasma to make it in a measurable level to the fetus.

    Now on the question of evolution, humans are born remarkably immune sensitive to their surroundings. We are born with an incomplete T cell repertoire that takes about 12 years to build, and we need maternal IgG to protect us for the first 6 months of life (longer if breast fed).

    Edit: Note there may be confusion about the status of "Monomer" when applied to IgG, IgG is a dimer of two identical heavy and light chains, but IgG is not found bound to other IgG so it is referred to as a monomer. A single IgM molecule is also a dimer, but it binds four other IgM in plasma to form an IgM pentamer.
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