Precipitin Arc
Definition
Curved line of antigen-antibody precipitate that forms at equivalence in immunoelectrophoresis. Shape and position identify the antigen.
- Technique
- Immunoelectrophoresis
- Formation point
- Zone of antigen-antibody equivalence
- Shape
- Curved line
- Diagnostic use
- Shape and position identify the antigen
Common questions
Why does the precipitin arc form specifically at equivalence?+
At equivalence the ratio of antigen to antibody is optimal for forming large, insoluble immune complexes; away from that ratio, in antigen or antibody excess, the complexes stay small and soluble, so no visible arc forms at those positions along the electrophoretic gel.
How does immunoelectrophoresis improve on simple double diffusion?+
Electrophoresis first separates the antigen mixture by charge before diffusion against antibody occurs, spreading overlapping antigens into distinct arcs along the gel, which resolves complex mixtures that would produce overlapping, hard-to-interpret bands in simple Ouchterlony diffusion alone.
Related terms
- Agarose
- Polysaccharide gel from seaweed. Large pore size, used for nucleic acids and large proteins. Standard for DNA submarine gels.
- Capillary Electrophoresis (CE)
- A separation technique that resolves DNA fragments by size through a polymer-filled glass capillary under an applied electric field. Single-base resolution is...
- Electrophoretic Mobility (μ)
- Velocity of a charged particle per unit electric field, μ = v/E. Depends on charge, size, shape and the viscosity of the...
- IEF (Isoelectric Focusing)
- Electrophoresis across an immobilised pH gradient. Each protein migrates until it reaches its pI and stops. Very high resolution; used in haemoglobin...
- Immunoelectrophoresis
- A two-step technique: proteins are first separated by electrophoresis in agar gel, then antiserum diffuses from a parallel trough and forms precipitin...
- Isoelectric Point (pI)
- pH at which a molecule (usually a protein) carries zero net charge and stops migrating. The basis of isoelectric focusing (IEF).
- Polyacrylamide (PAGE)
- Cross-linked synthetic gel with tunable, smaller pore size. Used for proteins and small DNA fragments.
- SDS-PAGE
- PAGE run with sodium dodecyl sulphate. SDS coats proteins with uniform negative charge, so separation is by molecular weight alone.