Precipitin quantitation (rocket immunodiffusion): In rocket immunoelectrophoresis, the height/length of the 'rocket' formed in gel is directly related to which variable?

Introduction / Context:
Rocket immunoelectrophoresis (Laurell method) is a semi-quantitative immunoassay that measures antigen concentration based on precipitin peak height formed during electrophoresis through an antibody-embedded gel.

Given Data / Assumptions:

• Agarose gel contains a uniform concentration of specific antibody.
• Antigen samples of varying concentration are loaded into wells.
• An electric field drives antigen migration and formation of precipitin arcs that resemble rockets.

Concept / Approach:
As antigen migrates into the antibody-rich gel, immune complexes form and precipitate at equivalence. Higher antigen concentration migrates farther before antigen becomes limiting, generating taller rockets. Thus, rocket height is proportional to antigen amount over the linear dynamic range.

Step-by-Step Solution:

Verification / Alternative check:
Calibration curves using standards (antigen vs. rocket height) yield linear plots within a validated range, confirming proportionality.

Why Other Options Are Wrong:

• Inverse relationships: incorrect for standard Laurell conditions.
• Dependence on antibody amount in the well: antibody is in the gel, not the sample wells.
• Independence from antigen concentration: contradicts the assay’s quantitative basis.

Common Pitfalls:
Out-of-range concentrations and diffusion artifacts can distort linearity. Always run standards and ensure proper gel casting and voltage control.

Final Answer:
Proportional to the amount of antigen placed in each well