The 11th International Conference on Lymphoid Tissues and Immune Reactions was held in Spa-Liege (Belgium), from 4 to 8 July 1993. The regular devotees refer to these conferences as the "Germinal Centre Conferences or GCC." In the 1960s, the germinal centres were the subject of such considerable study and speculation that a group of dynamic people decided to devote an international conference centered on that topic. This led to the fIrst GCC organized in Bern in 1966. Following the success of this initial meeting, further sessions have been organized at regular intervals and, over the years, the scope of the GCC has been broadened. Nowadays, the GCC conferences are dedicated to in vivo immunology and deal mainly with cellular, functional and molecular aspects of the lymphoid system. The credo of these conferences is "in vivo veritas," implying that the sole investigation of components (like molecules or cell populations) only gives a partial truth. Ultimately, the components have to be explored in their global context in order to see how they interact with other parts and how they are integrated as a whole to ensure the homeostasis of the immune system. In 1966, the GCC lasted 3 days and included 57 contributions "which were discussed at length. " The present conference lasted 4 days and accommodated 2 honorary lectures, 80 free papers and 209 poster presentations. Out of those presentations, a selection was performed by the Chairpersons to compose the present volume.