Which one of the statements concerning IgM is NOT true?

The statement regarding IgM being composed of four subclasses is not true. IgM is a unique antibody with a distinct structure and function. It is classified as a single immunoglobulin type and does not have subclasses like IgG, which has four different subclasses (IgG1, IgG2, IgG3, and IgG4).

This distinction is important because IgM plays a crucial role in the early immune response, acting as the first antibody produced by the immune system upon exposure to an antigen. Its pentameric structure, typically composed of five monomeric units, enhances its ability to bind to multiple antigens simultaneously, increasing its overall efficiency during the initial stages of the immune response. Moreover, IgM is effective at activating the complement system, which is part of the immune response that helps to clear pathogens. Understanding these characteristics of IgM helps clarify its vital role in immunology.