Principle of immunoelectrophoresis: In these assays, electrophoretic separation of mixture components occurs at what stage relative to the antigen–antibody reaction?

Introduction / Context:
Immunoelectrophoresis is a two-stage analytical technique used to resolve antigens by charge/size and then identify them by forming visible precipitin arcs with specific antibodies in gel.

Given Data / Assumptions:

• Mixture of antigens (e.g., serum proteins) is applied to a gel.
• An electric field first separates components according to electrophoretic mobility.
• Antiserum is then allowed to diffuse, forming arcs with cognate antigens.

Concept / Approach:
The defining feature is “separate first, immunoreact second.” This provides spatial resolution so each antigen meets antibody at a distinct location, enabling qualitative pattern recognition and semi-quantitative assessment.

Step-by-Step Solution:

Verification / Alternative check:
The classical scheme is documented in clinical immunology atlases: electrophoresis precedes the immunoreaction to create interpretable arc patterns.

Why Other Options Are Wrong:

• Reactions prior to separation or no electrophoresis: those describe immunodiffusion methods, not immunoelectrophoresis.
• Separating immune complexes after formation yields smeared or altered mobility, defeating the purpose.

Common Pitfalls:
Poor electrophoretic resolution or incorrect antiserum concentration can blur arcs; optimize voltage, buffer, and antibody dilution.

Final Answer:
Prior to reaction with antibody (separate first, then allow immunodiffusion)