Among the antibodies, which one is the least abundant in serum?

The least abundant antibody in serum is IgD. While IgG is the most prevalent antibody in serum and is crucial for long-lasting immunity, and both IgM and IgA play significant roles in the initial immune response and mucosal immunity, respectively, IgD is primarily found on the surface of B cells and only in small amounts in serum.

IgD’s main function appears to be in the initiation and regulation of B cell activation rather than in providing direct immune defense in circulation. Its low concentrations in serum indicate that it does not have a substantial role in systemic antibody-mediated immunity when compared to the other immunoglobulins. Thus, the presence of IgD in serum is minimal, making it the least abundant among the antibodies listed.