Question

Reactions between antibodies and antigens that are detected by precipitate formation in an agar gel are referred as

• A. Immunoprecipitation assay
• B. Immuno-diffusion assay
• C. Immuno-fluorescence
• D. Immuno-aggregation assay

Hint:

Immuno-diffusion assays require no specialized detectors because the precipitation line is directly visible to the naked eye at the zone of equivalence.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Antigen-antibody interactions are highly specific biochemical reactions used in diagnostic immunology.
When soluble antigens meet their complementary bivalent antibodies in an aqueous solution, they form cross-linked immune complexes.
When these complexes grow large enough, they lose solubility and precipitate out of solution as a visible line or precipitate.

Step 2: Detailed Explanation:
When this precipitation reaction is performed in a semi-solid agar gel matrix, it is called an immuno-diffusion assay.
In this assay, the antigen and antibody are placed in separate wells cut into the agar gel.
Both molecules diffuse radially through the gel pores toward each other.
As they diffuse, a concentration gradient is established for both the antigen and the antibody.
When the moving diffusion fronts meet, they form a visible precipitate line along the zone of equivalence, where the concentration ratio of antigen to antibody is optimal for cross-linking.
Common types of these assays include:
- Ouchterlony double immunodiffusion, where both the antigen and antibody diffuse outward from adjacent wells. This method is used to determine antigenic relationships.
- Single radial immunodiffusion (Mancini method), where the antibody is mixed uniformly into the agar gel, and the antigen diffuses from a central well. The diameter of the resulting precipitation ring is used to quantify the antigen concentration.
While immunoprecipitation assays also rely on precipitation, they are typically performed in liquid tubes to isolate proteins.
Assays performed within agar gels are classified as immuno-diffusion assays.

Step 3: Final Answer:
Reactions characterized by precipitate formation within an agar gel are immuno-diffusion assays.