Blocking serums consists of dilute serum from the same species used for production of the link antibody. They are usually used just prior to the incubation of the primary antibody. Blocking serums can be made of 1% to 20% depending on the tissue and antibody that are used. Blocking serums can be added to the washing solution if preferred, but is usually not necessary for most IHC techniques. Higher concentrations of blocking serums are used when the primary antibody has a high protein concentration but you also need to use a high concentration of the primary to obtain sufficient staining without excessive background.
