Purified IgG means the immunoglobulin G fraction has been separated from the rest of antiserum. The label conceals an important distinction, and a datasheet that does not specify which method was used is incomplete.
Two very different purifications
Protein A or Protein G purification captures IgG through its Fc region. It removes albumin, protease and non-IgG serum protein, and yields a solution of known concentration in mg/mL suitable for conjugation — but every IgG in the animal is captured, so the target-specific fraction may still be only a small percentage of the total. Protein A and Protein G differ in their affinity for host species and subclasses, which is why the host is always stated.
Antigen-affinity purification passes the serum over immobilised peptide and elutes only antibodies that bind it. The product is target-specific IgG, gives markedly lower background, and is the right choice for immunohistochemistry, conjugation and any quantitative ELISA. Yield is much lower, and the low-pH or chaotropic elution can damage the most avid clones.
Either way, purified IgG is stated in mg/mL, allowing real titration rather than the dilution-factor guesswork of whole serum.
Related terms and material
polyclonal vs monoclonal · blocking peptide · immunogen. Reference material: antibodies, labeled antibodies. Further reading: antiserum vs purified IgG.