Which technique plots the diameter of the precipitin ring against antigen concentration on a logarithmic scale?

• A. Fahey-McKelvey or the kinetic RID
• B. Mancini or the endpoint RID
• C. Immunoassay
• D. Ultrafiltration

Answer: (A)

The technique that plots the diameter of the precipitin ring against antigen concentration on a logarithmic scale is known as the Fahey-McKelvey or kinetic radial immunodiffusion (RID). This method is utilized in immunology to quantify the concentration of antigens based on the formation of immune complexes that precipitate during diffusion in an agar gel.

In this approach, the increase in the diameter of the precipitin ring is directly proportional to the concentration of the antigen in the sample. By using a logarithmic scale for plotting, the relationship becomes clearer and allows for more accurate interpretations, particularly in samples with wide-ranging concentrations. The kinetic aspect refers to measuring the diameter of the ring at various time points, enhancing the precision of the quantification process.

The other techniques mentioned do not employ this specific logarithmic plotting of precipitin ring diameters in their methodologies. The Mancini or endpoint RID does not focus on kinetic measurements but rather measures the diameter of the precipitin ring at a fixed endpoint, which does not involve a logarithmic scale for plotting antigen concentration. Immunoassay is a broader category of techniques used for various types of antigen and antibody detection, while ultrafiltration pertains to separation techniques rather than immunological quantification. Thus,