Precipitation Reaction
المؤلف:
APURBA S. SASTRY , SANDHYA BHAT
المصدر:
Essentials Of Medical Microbiology 2021
الجزء والصفحة:
3rd edition , p153-154
2025-08-14
629
Definition
When a soluble antigen reacts with its antibody in the presence of optimal temperature, pH and electrolytes (NaCl), it leads to formation of the antigen–antibody complex in the form of:
- Insoluble precipitation band when gel or agar containing medium is used (called immunodiffusion) or
- Insoluble floccules when liquid medium is used (called flocculation test).
Note: The results of immunodiffusion can be obtained quicker by carrying out the test in presence of electric current. This modification is called as immunoelectrophoresis. It has two variants such as countercurrent Immunoelectrophoresis (CIEP) and rocket electrophoresis.
Clinical Applications
Earlier, precipitation reactions were one of the widely used serological tests. However with the advent of simple and rapid newer techniques their application is greatly reduced. There are only limited situations where precipitation reaction is still in use; discussed below.
Slide Flocculation Test (for Syphilis)
It is used for serodiagnosis of syphilis, a sexually transmitted disease caused by Treponema pallidum.
- Procedure: When a drop of antigen is mixed with a drop of patient’s serum (containing antibody) on a slide, then the precipitates formed remain suspended as floccules
- Examples include VDRL (Venereal Disease Research Laboratory) and RPR (Rapid Plasma Reagin) tests.
Elek’s Gel Precipitation Test (Detecting Diphtheria Toxin)
The Corynebacterium diphtheriae strain isolated is streaked on to a medium containing a filter paper soaked with diphtheria antitoxin.
- If the strain is toxigenic, it produces the toxin, which diffuses in the agar, meets with the antitoxin and produces arrow-shaped precipitation band
- This test can also be used to know the relatedness between the strains isolated during an outbreak.
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