A precipitin is an
antibody
An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...
which can
precipitate
In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...
out of a solution upon antigen binding.
Precipitin reaction
The precipitin reaction provided the first
quantitative assay for antibody. The precipitin reaction is based upon the interaction of
antigen
In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune respons ...
with antibody leading to the production of
antigen-antibody complexes.
To produce a precipitin reaction, varying amounts of soluble antigen are added to a fixed amount of serum containing antibody. As the amount of antigen added:
* In the zone of antibody excess, each molecule of antigen is bound extensively by antibody and crosslinked to other molecules of antigen. The average size of antibody-antigen complex is small; cross-linking between antigen molecules by antibody is rare.
* In the zone of equivalence, the formation of precipitin complexes is optimal. Extensive lattices of antigen and antibody are formed by cross-linking.
* At high concentrations of antigen, the average size of antibody-antigen complexes is once again small because few antibody molecules are available to cross-link antigen molecules together.
The small, soluble immune complexes formed ''
in vivo
Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...
'' in the zone of antigen excess can cause a variety of pathological syndromes.
Antibody can only precipitate antigenic substrates that are multivalent—that is, only antigens that have multiple antibody-binding sites
epitope
An epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The epitope is the specific piece of the antigen to which an antibody binds. The p ...
s. This allows for the formation of large antigen:antibody complexes.
External links
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Biochemistry detection reactions
Immune system
{{Biochemistry-stub