A B cell switches from making IgM to making IgG. What happens to the antigen-binding site of the antibody?

1. IgM is made first, and as the primary response proceeds the antibody produced in a B cell can change to IgG, IgA or IgE. 2. That change of one antibody class to another is known as class switching. 3. The main differences between the classes lie in their heavy chains, which carry effector jobs such as crossing the placenta or entering secretions. 4. While the class of antibody changes, the specificity and the antigen-binding sites do not. 5. So the antibodies made after switching are still specific to the pathogen that stimulated the initial IgM response. 6. An option that widens or rebuilds the binding site would hand the B cell a new specificity, breaking that continuity. 7. An option that loses the site altogether would leave a secreted protein that binds nothing, which is not what class switching produces. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 21 "The Lymphatic and Immune System", section 21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies_