Quantitative Radial Immunodiffusion (RID – Mancini Technique) for Serum Immunoglobulin Assay
๐ฌ Principle & Mathematical Derivation
Radial Immunodiffusion (RID), developed by Giuliano Mancini, J. Carbonara, and J.F. Heremans in 1965, is a single diffusion technique in which a monospecific polyclonal antibody is incorporated at uniform concentration into molten agarose.
When antigen is pipetted into cylindrical wells cut into the gel, it diffuses radially outward in all directions. Initially, near the well margin, antigen is present in large excess ($Ag > Ab$), forming soluble antigen-antibody complexes. As the antigen diffuses outward, its concentration drops until it reaches the Zone of Equivalence, where the ratio of multivalent antigen to bivalent antibody forms an insoluble, high-molecular-weight crosslinked three-dimensional lattice. This produces an opaque, circular precipitin ring.
At diffusion equilibrium (the Mancini endpoint method, typically 48 to 72 hours), all available antigen has migrated out of the well and reacted with antibody. Mathematically, the area of the precipitin ring ($pi r^2$) is directly proportional to the total amount of antigen applied:
Where $d$ is the outer diameter of the precipitin ring in millimeters, $m$ is the slope of the calibration line, $[text{Ag}]$ is the antigen concentration, and $c$ is the intercept on the y-axis (the square of the well diameter, $d_0^2$).
๐ Step-by-Step Laboratory Protocol
- Gel Preparation: Prepare a $1.2%$ solution of agarose in PBS (pH 7.4, with $0.02%$ sodium azide preservative). Boil until clear, then cool in a water bath to exactly $50^circtext{C}$ (higher temperatures denature antibody; lower temperatures cause premature gelling).
- Antibody Incorporation: Add monospecific goat or rabbit anti-human IgG antiserum to the agarose at a pre-calibrated dilution (typically $1:20$ to $1:50$). Gently invert to mix without generating bubbles, and pour immediately onto a level glass slide or RID plastic plate ($1.5,text{mm}$ uniform gel thickness). Allow to set at room temperature for 30 minutes.
- Well Punching: Using a sharp $3.0,text{mm}$ gel punch and template, punch 4 to 6 clean cylindrical wells spaced at least $15,text{mm}$ apart. Remove the agar plugs by gentle vacuum suction.
- Sample Loading: Using a calibrated precision micro-pipette, deliver exactly $5.0,mutext{L}$ of standard IgG calibrators ($25,mutext{g/mL}$, $50,mutext{g/mL}$, $100,mutext{g/mL}$, $200,mutext{g/mL}$) and unknown patient serum into designated wells. Avoid touching the well rim.
- Incubation: Place the plate horizontally inside a moist, sealed humid chamber (petri dish with wet filter paper) and incubate at room temperature ($20-25^circtext{C}$) for 48 hours to reach complete equilibrium.
- Ring Measurement: Place the plate on an illuminated RID viewer with a graduated eyepiece micrometer or dark-field magnifying viewer. Measure the ring diameter ($d$) to the nearest $0.1,text{mm}$ in two perpendicular directions and record the mean diameter.
