Excitability as a Design Principle in the Immune System

Growing datasets and mechanistic detail in immunology have outpaced the development of unifying concepts. Such concepts are required to explain the primary goals of immune circuits - strong response to pathogens, tolerance to self, and prevention of collateral damage. A principle that achieves these goals across diverse immune circuits could unify our understanding of the immune system. Here, we propose that excitability, a concept from dynamical systems, serves this role. We screen thousands of circuits to identify those that generate excitable dynamics, and find a single robust design. We scan the human immune network to find this circuit architecture in dozens of innate and adaptive subsystems. We provide evidence for excitability in data on longitudinal responses to SARS-CoV-2. Similar motifs underlie T cell activation, autoimmune flares, and tumor immune responses. This conserved motif provides therapeutic targets and suggests that excitability is a core design principle of immunity, bridging molecular and cellular levels.