TEVA says the "list of questions" is "short", but that does not mean they're easy!
Please correct me if I'm wrong, but my recollection is that in the internal discussions at the FDA, the OGD was the division that has been pressing for standards stricter than the five criteria specified in the response to the citizens petitions. The immunogenicity questions are their domain, and such questions may not by any means be simple for Teva to answer.
From the Teva news release:
"Prior to potential final product approval, the Office of Generic Drugs requires responses to a short list of questions, which Teva intends to respond to in the near future."