The basics of binding: mechanisms of antigen recognition and mimicry by antibodies.

New insights into the nature of antigen-antibody recognition have been gained through X-ray crystallographic studies of immune complexes. In particular, it has been demonstrated that water molecules form an extended network bridging antigen and antibody, and are essential in achieving shape and chemical complementarity between their interacting surfaces. This finding has important implications for the energetics of the association reaction. Recently, X-ray data on the complex between a peptide hormone and an anti-anti-idiotypic antibody have been obtained. This has relevance to the structural basis of antigen mimicry by antibodies. The conformation of the bound peptide was found to be very similar to that of an antibody complementarity determining region loop, providing a direct structural explanation for how antigen mimicry by anti-idiotypic antibodies might occur.