Two products sit next to each other in most immunology catalogs and get confused all the time: gamma globulin and antiserum. One is a purified protein fraction from normal animals. The other is whole serum from an immunized one. They're used together in almost every immunoassay, and knowing which is which saves you from ordering the wrong bottle.
Gamma globulin
The name comes from serum electrophoresis, where immunoglobulins migrate in the slowest, "gamma" band. A gamma globulin fraction is that band recovered from normal serum, mostly IgG with some IgM and IgA, from animals that weren't deliberately immunized. The antibodies in it are the animal's ordinary repertoire, so the product isn't against anything in particular, and that is the point: it is species-specific immunoglobulin with no defined specificity.
It's prepared by salt fractionation (ammonium sulfate or sodium sulfate precipitation), sometimes followed by ion exchange, and sold as a lyophilized powder that keeps for years. The uses follow from what it is. It's the calibrator and standard for assays that quantify immunoglobulin of that species. It's a blocking protein in immunoassays and immunohistochemistry, where immunoglobulin from the secondary antibody's host saturates non-specific sites. It's an isotype and Fc control in flow cytometry, cell culture and in vivo work, where the low-endotoxin grades matter (see low-endotoxin gamma globulins). And it's the immunogen for raising anti-species antibodies, which brings us to antisera.
Antiserum
An antiserum is the serum of an animal that has been immunized with a specific antigen and now carries a high titer of antibody against it. The antibodies are polyclonal, recognizing many epitopes on the antigen, and they're still in their native serum matrix along with albumin, complement and everything else. The workhorse antisera are the anti-species immunoglobulins, goat anti-human IgG, rabbit anti-mouse IgG and so on, raised by immunizing one species with the gamma globulin of another. These are the secondary reagents that detect a primary antibody in ELISA, Western blot and IHC, and the precipitating reagents in immunodiffusion and nephelometry.
Antisera are sold whole, as an IgG fraction, or affinity purified against the immunogen, in rising order of specificity and price. Whole antiserum is fine for precipitation and agglutination. Affinity-purified antibody is what you want for detection with low background.
How the two fit together
The gamma globulin of one species is the antigen for the antiserum of another, and it's also the calibrator that the antiserum is later used to measure. In a sandwich assay for human IgG, purified human gamma globulin is the standard and goat anti-human IgG is the detector. In an IHC protocol, normal serum or gamma globulin from the secondary's host is the block, and the antiserum against the primary's host is the secondary. Once you have the species straight the pairing is obvious.
We supply lyophilized gamma globulin fractions from rabbit, bovine, goat, sheep, pig, mouse and rat serum, a format that suits diagnostic-reagent manufacturing and raising anti-species antisera. The purification chemistry behind them is in IgG purification methods. If you need a species or a bulk quantity that isn't listed, contact us.