A gamma globulin preparation can be 99% pure IgG and still wreck a cell-culture experiment. The culprit is endotoxin, lipopolysaccharide from Gram-negative bacteria that rides along with proteins purified from serum, survives sterile filtration, and activates immune cells at picogram concentrations. For an ELISA blocking step it doesn't matter. For a macrophage culture or a mouse injection it can be the whole result.
What endotoxin does at low doses
Lipopolysaccharide binds Toll-like receptor 4 on monocytes, macrophages, dendritic cells and endothelium and triggers release of TNF-α, IL-1, IL-6 and other mediators. In culture that shows up as activation, cytokine production, altered proliferation or death, all of it attributed to whatever you added the protein to test. In an animal it produces fever, leukocyte changes and, at higher doses, shock. The response is sensitive enough that the cells will detect endotoxin no protein assay would.
How much is too much
Endotoxin is measured by the Limulus amebocyte lysate (LAL) assay in endotoxin units (EU) and reported per milligram of protein. Where a reagent's endotoxin isn't controlled, tens of EU/mg is common. A low-endotoxin grade is typically specified at or below 1 EU/mg, and sensitive primary cell work may need less than that. Work out the dose your cells or animals will actually see, protein concentration times EU/mg, and compare it with the roughly 0.1 EU/mL at which monocytes start to respond.
Where the low-endotoxin grade matters
Immune cell culture, first of all: anything with monocytes, macrophages, dendritic cells or PBMCs, including co-cultures and cytokine-release assays. In vivo work, where passive immunization, IVIG models, Fc-receptor studies and pharmacokinetics all have readouts endotoxin can confound, and where it can harm the animal. Antibody and vaccine development, where an inflammatory response in a preclinical study has to be attributable to the product and not to a contaminant. And isotype and Fc controls, because a control that carries endotoxin isn't controlling for much.
For blocking, diluents, calibrators and Western blotting, standard gamma globulin is fine and the low-endotoxin grade buys you nothing.
How it's made
Endotoxin control starts with clean serum and pyrogen-free buffers and glassware, continues through purification steps that separate LPS from protein, and ends with LAL testing of the finished lot. The number on the lot's Certificate of Analysis is what to check. "Sterile" on its own says nothing about endotoxin, because filtration removes bacteria but not their lipopolysaccharide.
We purify gamma globulin fractions from rabbit, bovine, goat, sheep, pig, mouse and rat serum as lyophilized powder, with endotoxin reported per milligram where the specification applies. If your work needs a tighter limit than the catalog grade, tell us the target and we can often test a lot against it before it ships. For how these fractions fit into a research workflow, see biological reagents for academic and translational research.