|MadSci Network: Immunology|
An interesting question! In some ways you already answered it. When the cells are fused (usually by polyethylene glycol) the resulting fused cell does actually become tetraploid - but only for a short time. The fused cells are actually quite unstable and begin to throw out chromosomes in the hope of restoring their diploid status. Of course, sometimes they don't survive this process and they can also throw out the chromosome that has the rearranged Ig gene on it, resulting in the loss of antibody production. So they may not be dipoid nor tetraploid but they will be somewhere between the two.....See 'Antibodies' by Harlow and Lane, an excellent source of all things antibody!
Try the links in the MadSci Library for more information on Immunology.