How can our immune system defend us against aggressors so diverse such as viruses, parasites, fungi and tumors? The secret lies in the large number of clones of T and B lymphocytes, each of which expresses a particular specific receptor. Until a few years ago, deciphering the complexity of this vast repertoire was considered impossible. A “Rosetta stone”, or a key for decoding, was missing in order to “translate” and understand this “language” in all its complexity.