The immune system is an anatomically structured, orchestrated interaction of different cell types that communicate via a large number of receptors recognizing both soluble and cellular ligands. Recent technological advances now allow large-scale measurements for better appreciation of this complexity. Despite these advances, only a few immunological parameters are routinely measured in clinical practice. The authors believe that these measurements are insufficient to describe the immune function of individual patients and thus cannot be used to evaluate immune-mediated diseases or response to therapy. Our current knowledge of immunology comes largely from work in murine model systems where the immune system has been characterized in great detail. This impressive volume of knowledge has proven to be difficult to translate into novel therapies in humans; one reason for this is the lack of large-scale immune monitoring allowing for systems-wide analysis of the human immune system. The authors propose a systems approach to immunology, where the focus is moved from analysis of individual cell types towards more integrated studies of the entire immune system. Exercising 'systems immunology' in preclinical research, during drug development and in patients undergoing therapies affecting the immune system, will enable us to improve clinical results through personalized medicine and help to define clinically relevant patterns of immune reactivity.