According to the 'self-nonself' framework (left), the immune system is mainly a system for targeting and killing foreign bodies. Interfaces, such as the gut lumen, belong to the ‘outside’ of the organism, and boundaries are strict and fixed. According to the newly emerging 'immunological individual' framework (right), the immune system can eliminate self and nonself elements, it can tolerate self and nonself elements, but it also reinforces the cohesion between bodily constituents. In this framework, boundaries are constantly being redefined by the action of the immune system. Image credit: Wiebke Bretting (CC BY 4.0).