IgM properties: which statement is false?

IgM properties: which statement is false?

• A. Has a J chain.
• B. Is a primary response antibody.
• C. Can cross the placenta.
• D. Fixes complement.

Answer: (C)

IgM is the antibody you see first in a primary immune response, and it mainly exists as a secreted pentamer linked by a J chain. That J chain is what allows several IgM units to come together into a pentamer, increasing its overall avidity and making it very effective at kicking off the classical complement pathway. Because IgM is a large pentamer, it cannot cross the placenta; the placenta mainly transports IgG through the FcRn receptor. That’s why the statement about crossing the placenta is false. The other properties are correct: it has a J chain, it’s produced early in a primary response, and it fixes complement.

IgM is the antibody you see first in a primary immune response, and it mainly exists as a secreted pentamer linked by a J chain. That J chain is what allows several IgM units to come together into a pentamer, increasing its overall avidity and making it very effective at kicking off the classical complement pathway. Because IgM is a large pentamer, it cannot cross the placenta; the placenta mainly transports IgG through the FcRn receptor. That’s why the statement about crossing the placenta is false. The other properties are correct: it has a J chain, it’s produced early in a primary response, and it fixes complement.