The cells of the immune system generate a large variety of binding sites which differ in their binding specificities and can therefore react specifically with a large variety of ligands. These binding sites are part of receptor molecules, enabling the system to react to the universe of antigens. The classical antigen receptor is the antibody molecule, and accord ingly the first session of this colloquium deals with a classical sub ject, namely antibody structure. Dramatic recent advances in this field make it possible to interrelate primary and three-dimensional struc ture both to each other and to function, i.e. the binding of antigen and possible reactions occurring in the antibody molecule upon antigen binding. The latter point is of particular interest since it may be relevant not only for effector functions of antibodies such as the binding of complement, but also for the triggering of a lymphocyte through its antibody receptor for antigen.