Neutralizing antibodies are only the tip of the iceberg. Neutralizing antibody responses are induced as a result of a cascade of tightly regulated interactions between different cells of the innate and adaptive immune system located in distinct compartments. Any event that modulates the repertoire, activation state, or migration pattern of immune cells will therefore potentially influence the risk of patients developing inhibitors. (Figure adapted from license-free figure 5348288 from www.fotolia.de)