Protein A binds antibodies from several species.  So if you incubate your rat-IgG-ProtA complex in excess human IgG solutions such as plasma, the human IgG will displace the rat IgG in the complex.

Protein A binds antibodies from several species.  So if you incubate your rat-IgG-ProtA complex in excess human IgG solutions such as plasma, the human IgG will displace the rat IgG in the complex.

Rat IgG is a poor binder to Protein A; binds much better to Protein G:

https://www.gelifesciences.com/gehcls_images/GELS/Related%20Content/Files/1314735988470/litdoc18116844p20_20130407222048.pdf

Thank you very much Edmund and Lars.

I will do the targeting experiment in washed PBMCs rather than whole blood to minimise any human IgG competing off the bound rat IgG.

Much appreciated.

After loading the beads, I'd try to crosslink the complex (glutaraldehyde, but a bit risky) or better use

a) biotinylated antibody on streptavidin carrier (virtually covalent)

b) NHS or CNBr activated carriers.

c) carboxy beads activated with EDC/NHS on the fly to couple your ABs

Which sub type of Rat IgG you want to get it bind by Protein A? Protein A binds with very low affinity to Rat IgG1 and there is no affinity for Rat IgG2a, IgG2b, and IgG2c on the other hand Protein A bind firmly to human IgG1, IgG2 and IgG4.  Similarly Protein G have much more binding affinity for human IgG1, IgG2, IgG3 and IgG4  than Rat IgGs except IgG2a.