Which antibody class has no known function other than acting as a receptor on naive B cells?

1. Of the five antibody classes, only two can function as the antigen receptor for naive B cells, IgM and IgD. 2. Mature B cells that leave the bone marrow express both, and the two antibodies have the same antigen specificity. 3. Of that pair only IgM is secreted, and no other nonreceptor function for IgD has been discovered. 4. IgA and IgG are secreted antibodies with clear jobs in exocrine secretions and in blood, so neither fits the description. _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_