Antiserum is the whole serum collected from an animal after immunisation: blood is allowed to clot, cells and fibrin are removed, and what remains is the liquid fraction. Crucially, it contains the antibodies raised against your immunogen and the animal's entire pre-existing immunoglobulin repertoire, plus albumin, complement components and serum proteases.
Why the format matters
Target-specific antibody is typically only a few percent of total IgG in an antiserum. For applications where excess irrelevant protein is harmless — many ELISA and dot-blot uses, or immunohistochemistry at high dilution — antiserum is the cheapest and most avid option, and its high total protein even acts as its own carrier. For anything requiring conjugation, low background, or a defined protein concentration, the extra material is a liability: labelling chemistry will modify all IgG indiscriminately, and non-specific serum binding raises background.
Titre for an antiserum is expressed as the working dilution giving acceptable signal, not as mg/mL of specific antibody, because specific concentration is generally unknown. Sodium azide or glycerol is normally present as preservative, which is incompatible with HRP conjugates and should be checked before use.
Related terms and material
purified IgG · polyclonal vs monoclonal · species reactivity. Reference material: antibodies catalog. Further reading: antiserum vs purified IgG.