Final answer:
The characteristic of antibodies within the ABO system that is NOT true is that they are IgM class antibodies.
Step-by-step explanation:
The characteristic of antibodies within the ABO system that is NOT true is C. Antibodies within the ABO system are NOT IgM class antibodies. They are IgM and IgG class antibodies. IgM antibodies are the first antibodies produced in response to an infection or foreign antigen. They are usually pentameric in structure and are efficient in activating the complement system. IgG antibodies, on the other hand, are smaller and more specialized antibodies that are produced later in the immune response. They play a key role in long-term protection against pathogens.