Are all antibodies against a common antigen identical?
Alright, there are two major questions:
(a) Are all the antibodies that are produced against a specific antigen necessarily identical?
Not in most cases. Antigens can either be monovalent or polyvalent, meaning that they can either have one epitope or various different epitopes (an epitope is the site of the antigen that binds to a T-cell receptor or a B-cell receptor). Most antigens are polyvalent, which implies that a single antigen may interact with several different T and B cell lineages. Each B cell lineage is only able to produce and export one type of antibody. Because the antigen will interact with different lineages, it will promote the production of different types of antibodies. These multiple antibodies produced against one antigen are called polyclonal antibodies, since they arise from multiple clonal lineages. When the antigen only has one kind of epitope, it will induce the production of only one type of antibody, which are called monoclonal antibodies, because they arise from only one clonal lineage.
(b) Do the antibodies produced against a certain antigen differ between members of the same species or between species?
Yes, in both cases. There is an important genetic variability surrounding immunoglobulins and T cell receptors.
Reference: Karp, G. (2009). Molecular and Cellular Biology: Methods and Concepts. Wiley. New York.