Counterimmunoelectrophoresis principle: During counterimmunoelectrophoresis in gel, how do antigen and antibody migrate under an applied electric field?

Introduction / Context:
Counterimmunoelectrophoresis (CIE) accelerates antigen–antibody precipitation by applying an electric field across wells cut in an agarose gel. It is useful for rapid detection of certain microbial antigens in clinical samples.

Given Data / Assumptions:

• Separate wells contain antigen and antibody.
• An electric field is applied so that reagents migrate according to net charge at the running buffer pH.
• Precipitin lines form where migrating antigen and antibody meet at equivalence.

Concept / Approach:
The hallmark of CIE is opposing migration—driving antibody and antigen toward each other. This contrasts with passive immunodiffusion where movement relies solely on diffusion and takes longer. Adjusting buffer pH ensures opposite net charges for optimal convergence.

Step-by-Step Solution:

Verification / Alternative check:
Clinical protocols (e.g., for bacterial capsular antigens) specify rapid arc formation within minutes when current is applied, confirming opposing migration.

Why Other Options Are Wrong:

• Parallel or same-direction migration: would decrease encounter rate and is contrary to CIE design.
• Stationary antigen or random diffusion only: contradicts the use of electrophoresis.
• Both toward anode: charge-dependent migration is set to oppose, not match.

Common Pitfalls:
Improper buffer pH can yield weak or absent arcs; verify charge states so antigen and antibody migrate oppositely.

Final Answer:
They are driven toward each other to hasten precipitin line formation