My goal is to know what is the minimum volume and concentration of monoclonal antibody that is able to react with a specific volume and concentration of pure antigen?
IgG have 2 binding sites for antigen, so if the antigen (G) has only one binding site for the antibody (B), you look at two different equilibria:
B + G ⇄ BG and BG + G ⇄ GBG, each reaction with its own equilibrium constant. For IgM, the situation is, of course, more complex.
Where also the antigen (Mdr1) has two binding sites for the antibody (C219), a formal analysis is complicated, but I found (DOI:10.1046/j.1432-1327.1999.00643.x) that the data can be empirically described by the Sachs-Welt equation for a random-bi model (DOI:10.1172/JCI105512).
Recall, that for the antibody/antigen reaction you need not only look at the affinity, but also the avidity. I have explained that in DOI:10.1007/978-3-319-19920-7_11.
Another approach you could use is Bio-Layer Interferometry (BLI). Purified antibodies and antigens will help you calculate the dissociation constant for their interaction. You can also determine the minimum concentrations that allow for a detectable binding.
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