Antibody cross-linking can occur when target antigen epitopes in the tissue sample are shared with other proteins that exist naturally in the tissue sample. This results in background staining. The most effective method of minimizing this type of non-specific staining is incubation of the tissue section in a blocking serum prior to incubation in the primary antibody.
Blocking serums consist of dilute serum from the same species used for production of the secondary antibody. Blocking serums are applied 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. Higher concentrations of blocking serums are used when the primary antibody has a high protein concentration. Blocking serums can also be added to buffer wash solutions if preferred, but this is usually not necessary for most IHC staining techniques.