Which of the following represents the relative concentration of immunoglobulins in decreasing order in normal serum?

The correct answer is based on the typical distribution of immunoglobulins in normal human serum. In healthy individuals, immunoglobulins are present in the following approximate concentrations, expressed in mg/dL: IgG (around 700-1600 mg/dL), IgA (about 70-400 mg/dL), IgM (approximately 40-250 mg/dL), IgD (around 0.5-3 mg/dL), and IgE (generally less than 0.5 mg/dL).

This ranking reflects the predominant role of each immunoglobulin in the immune response. IgG is the most abundant and plays a crucial role in long-term immunity and neutralization of pathogens. IgA is important in mucosal immunity and is found in secretions such as saliva and tears. IgM is typically the first antibody produced in response to an infection. IgD has a less well-defined function but is involved in the activation and regulation of B cells. IgE is associated with allergic responses and parasitic infections, and is present in very low concentrations.

Therefore, the order in the correct answer aligns with their concentrations and roles in immune defense in normal serum, accurately reflecting the typical immunoglobulin profile.