Why is generation of the IgM class of antibodies advantageous upon first exposure to a new antigen?
A. One molecule of IgM has 10 binding sites and is capable of binding 10 com-pletely different antigens.
B. Initial antibody production is limited and IgM is the most efficient antibody class for binding and clearing antigens.
C. The assembly and activation of IgM molecules take less energy than the other classes of antibodies, and this allows IgM to have a more rapid response.
D. IgM molecules have a longer life span (4 to 5 years) compared with all other classes of antibody molecules, whose life spans range from only hours to days.
B
When B lymphocytes are first exposed to an antigen, it takes days before they become plasma cells and first begin to make antibodies. They also make very few molecules of antibody to start with, which is why people often get sick on first exposure to an organism. Because IgM has 10 binding sites, even though there are only small numbers of these molecules made at first, each one can bind up to 10 molecules of the antigen (the same antigen, not 10 different ones), making the clearance of antigen more efficient than with a bivalent antibody.
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