Which class of immunoglobulins is secreted as a pentameric structure and constitutes the primary antibody produced during the initial primary humoral immune response to an invading antigen?

💡 Solution & Explanation

Immunoglobulin M is the largest antibody isotype, typically circulating as a pentamer, and is the first immunoglobulin expressed on B cells and secreted during primary immune responses. Immunoglobulin G is the most abundant serum antibody crossing the placenta, Immunoglobulin A protects mucosal surfaces, and Immunoglobulin E mediates allergic hypersensitivity reactions. Measuring immunoglobulin levels helps diagnose immune deficiencies.