Precipitin Reaction
Definition
The formation of an insoluble antigen-antibody complex (precipitate) when soluble antigen and antibody meet at equivalent concentrations. The Ouchterlony double-diffusion test and the ring precipitin test both rely on this reaction to confirm species identity of a biological stain.
- Type
- Antigen-antibody precipitate formation
- Condition
- Equivalent antigen and antibody concentrations
- Common tests
- Ouchterlony double diffusion, ring precipitin test
- Purpose
- Confirms species of origin of a biological stain
- Category
- Confirmatory serological test
Common questions
Why does the precipitin reaction fail outside the equivalence zone?+
When antigen or antibody is present in large excess, small soluble complexes form instead of a visible lattice, so no precipitate line appears. Analysts sometimes dilute the extract to bring the reaction back into the equivalence zone where lattice formation and visible precipitation occur.
Does a positive precipitin reaction identify the individual who left the stain?+
No, it confirms species only, typically human origin. Individual identification requires a separate step such as STR DNA profiling performed on the same or an adjacent sample after species has been confirmed.
How is the Ouchterlony test read?+
Antigen and antiserum diffuse toward each other through a gel from separate wells, and a visible line forms where they meet at equivalent concentration. The line's position and shape are compared against a known positive control run in an adjacent well on the same plate.
Related terms
- Ouchterlony Double Diffusion
- A gel diffusion technique in which antigen and antibody placed in separate wells in an agar gel diffuse passively toward each other....
- Anti-Human Serum
- Antibodies raised in a rabbit (or goat) by repeated injection of human serum. The rabbit's immune system makes IgG against human serum...
- Antibody (Immunoglobulin)
- A Y-shaped glycoprotein produced by B lymphocytes that binds a specific antigen at its variable regions. Antibodies used as forensic reagents are...
- Antigen
- A molecule recognised by the adaptive immune system through specific binding to an antibody or T-cell receptor. Defined by immunogenicity (capacity to...
- Antigen-Antibody Reaction
- The specific non-covalent binding between an antigen (a molecule capable of eliciting an immune response) and the antibody produced against it. In...
- Antiserum
- Serum collected from an immunised animal containing antibodies directed against a specific antigen or group of antigens. In species identification, antiserum raised...
- COI Barcoding
- DNA-based species identification using a standard 658 bp segment of the mitochondrial cytochrome oxidase subunit I gene. Sequences are matched against reference...
- Counter-Immunoelectrophoresis (CIE)
- An accelerated form of gel diffusion in which an electric current drives antigen and antibody toward each other rather than relying on...
- Cross-Reactivity
- The capacity of an antibody raised against one analyte to bind structurally related compounds. In RIA, cross-reactivity is the main driver of...
- Cyt-B PCR
- Amplification and sequencing of the mitochondrial cytochrome-b gene. The reference marker for species identification in degraded and trace samples.
- ELISA (Enzyme-Linked Immunosorbent Assay)
- A plate-based immunoassay format in which a target antigen is captured by an antibody bound to a solid surface, detected by a...
- Immunoelectrophoresis
- A two-step technique: proteins are first separated by electrophoresis in agar gel, then antiserum diffuses from a parallel trough and forms precipitin...
Explained in these topics
- Determination of Species of Origin from Blood and Body FluidsAntigen-antibody reaction in which soluble antigen (the unknown blood protein) and specific antibody (anti-species serum) form a visible insoluble complex at t...
- The Role of Immunology in Forensic Science
- The Precipitin Test for Species IdentificationThe visible precipitation of insoluble antigen-antibody complexes that forms when soluble antigen and its specific antibody meet at approximately equivalent co...