According to the manual (see link below) of Pierce® Crosslink Immunoprecipitation Kit, 10 ug of IgG are used for 20 ul of Protein A/G agarose beads. No blocking of excess Protein A/G was mentioned. You can block the Protein A/G with non-immune IgG, after removing your specific antibody, and then cross-link all IgG molecules to the beads. This may prevent subsequent binding of nonspecific IgG from you lysates and co-elution of this nonspecific IgG with your specific antigen. Alternatively, you can preclear your lysates with Protein A/G agarose beads, before incubating them with the specific antibody-agarose beads.http://www.piercenet.com/instructions/2162134.pdf
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