Hapten is s protein fragment that functions as an antigen when coupled to a protein carrier molecule (see diagram below).  The resulting hapten-carrier conjugate used in this experiment is injected into mice, causing an immune response in the form of secreted antibodies.  The antibodies are then harvested from the mice and placed into an artificial environment so they are able to continue growing.  Through the method of X-ray crystallography, scientists are able to visualize the germ line amino acid sequences on the light and heavy chains of the antibodies, including the hapten binding sites.  By visualizing the antibodies at different maturation points, scientists observed that the germ line DNA that codes for these amino acids undergoes mutations, giving rise to greater antibody-antigen affinity.   The experiment provides important insights into our understanding of the binding of antibodies to antigens.