Our immune system is a complex array of cell types and signaling molecules. Cytokines—small, secreted proteins—are particularly critical signal carriers in the immune system. So important, in fact, that there have been over a million scientific studies about cytokines. But what if immunologists had a ‘periodic table’ of cytokines, like the one chemists use to reference the elements? One team of researchers set out to do just that. First, they systematically measured how every major gene in every major immune cell type responds to every major cytokine. Then they developed the companion software “Immune Response Enrichment Analysis” to assess cytokine activity and immune cell responses. This dictionary can help generate new hypotheses and unravel the roles of specific cytokines in signaling networks and already revealed that the complexity of cytokine responses and the adaptability of immune cells is greater than previously imagined. More work is needed to add other cell types, even beyond the immune system, as well as test variations in dose, duration, or physiological context. But overall, this study and the new Immune Dictionary is a valuable new tool for researchers exploring the immune system.