Suppose a scientist wants to make antiserum specific for IgG. They inject
a RABBIT with purified MOUSE IgG and get an antiserum thats reacts with MOUSE IgG and other mouse isotopes. Why did this happen? How can she make antiserum specific for RABBIT IgG?
This is a fairly classical question for an immunology class, and I have seen ones similar to this show up in numerous final exams etc. What the person who asked you is looking for is for you to show an understanding of the fact the IgG (and all immunoglobulins) are proteins. Because immunoglobulins/antibodies are so often focused on as being the thing that recognised protein antigen, it is common for students to overlook the fact that they are themselves proteins, and thus can be antigens.
In the question, when you inject IgG from one species ...
Explores how antibodies against other antibodies can be made, incorporating a thorough discussion of self vs non-self recognition.