Quantitation of immunoglobulin classes in serum: which immunodiffusion method is classically used to measure concentrations (e.g., IgG, IgA, IgM)?

Introduction / Context:
Measuring serum immunoglobulin concentrations is essential in diagnosing immune deficiencies, monoclonal gammopathies, and monitoring therapy. While nephelometry and turbidimetry are common today, single radial immunodiffusion (SRID) remains a foundational technique and a frequent exam topic.

Given Data / Assumptions:

• SRID (Mancini) = single diffusion, radial, two-dimensional area proportional to antigen concentration.
• Agarose gel contains specific anti-immunoglobulin antibodies.

Concept / Approach:
In SRID, antigen (serum Ig) diffuses radially from a well into antibody-containing gel. A ring of precipitation forms at equivalence. The square of the ring diameter is proportional to antigen concentration, allowing construction of a standard curve for quantitation of Ig classes.

Step-by-Step Solution:

Verification / Alternative check:
Immunology manuals list SRID for class-specific Ig measurement, with results compared to calibrators of known concentration.

Why Other Options Are Wrong:

• Ouchterlony (double diffusion in two dimensions) is qualitative/semiquantitative for antigenic relationships.
• Oudin (double diffusion in one dimension) is older and not the standard for precise quantitation.
• Single diffusion in one dimension is not the classical quantitative SRID technique.

Common Pitfalls:

• Mixing up diffusion dimension terminology; “radial” implies two-dimensional spread from a point source.

Final Answer:
Single diffusion in two dimensions (single radial immunodiffusion, Mancini method)