The major goal of this research program is to understand the detailed chemistry of interaction of an antibody with several related but chemically and conformationally different antigens. The secondary goals are to understand the diversity of the immune response by studying the structures of different antibodies which recognize the same antigen. The final goal is to understand the nature of anti-idiotypic cascade and how such antibodies can be used for biomedical and biotechnological purposes. Specifically the main objectives are to determine the high resolution three-dimensional structures of a high affinity anti-progesterone monoclonal antibody Fab' and its complexes with nine different steroids. Such detailed information will significantly add to the growing knowledge concerning the structural basis of immune recognition. This study differs significantly from other antibody structure determinations in that it addresses the key question of how a single antibody combining pocket can accommodate different antigens. This issue is central not only to understanding the diversity of antibody-antigen recognition but also to provide the much needed structural data required for the manipulation of antibody combining site by site-directed mutagenesis or bioengineering to design novel antibodies of catalytic function or for therapeutic, biological or biotechnological use. The structure determination of different anti progesterone antibody Fab's and their complexes will address the range and diversity of the immune response to a given antigen as well as to correlate the differences in their fine specificities with the three- dimensional structures. The structural basis and mechanism of anti- idiotypic response to a given antibody will be addressed by the structural determination of the DB3 with its corresponding anti idiotypic antibody. The uses of such anti-idiotype antibodies are of fundamental significance in biology and medicine including the possible use of anti-idiotypes as immunogens or vaccines to generate internal image antibodies to produce antibodies reactive to a given antigen without prior immunization with the antigen in question. The fascinating issue of the structural basis of such antibody generation will be answered by the structure determination of the Ab2 and Ab3 antibodies generated against the DB antibodies. All the Fab structures will be determined by x-ray crystallography using molecular replacement techniques or if necessary standard isomorphous replacement methods. Cryocrystallograph at ~-150oC will be used to increase crystal lifetimes and enhance the high resolution structure analyses.