<qa>  What is the adaptive immune system?The adaptive immune system is one of the only two biological systems capable of continuously learning and memorizing its experiences. This is a highly complex, distributed system, in which pathogen recognition, decision-making and action are performed by an interacting network of diverse lymphocytes. Immune learning and memory are embedded in the dynamical states of the complete lymphocyte repertoire, and cannot be understood by studying the behavior of single cell types. This complexity, further increased by the non-linear behavior of each component, can only be elucidated by using theoretical tools to complement experimental and clinical studies. Needless to say, many aspects of the deregulation of lymphocyte clones are not evident in the phenotype of the single cell but rather in the population dynamics of a whole clone or many clones of cells, as in B cell lymphomas. Such aspects are best elucidated by studies of the population dynamics and genetics of the relevant B cell clones. 