Management in pervasive systems cannot rely on human intervention or centralised decision-making functions. It must be devolved, based on local decision-making and feedback control-loops embedded in autonomous components. We have previously proposed the self-managed cell (SMC) as an architectural pattern for building ubiquitous applications, where a SMC consists of hardware and software components that form an autonomous administrative domain. SMCs may be realised at different scales, from body-area networks for health monitoring, to an entire room or larger distributed settings. However, to scale to larger systems, SMCs must collaborate with each other, and federate or compose in larger SMC structures. This paper discusses requirements for interactions between SMCs and proposes key abstractions and protocols for realising peer-to-peer and composition interactions. These enable SMCs to exchange data, react to external events and exchange policies that govern their collaboration. Dynamically customisable interfaces are used for encapsulation and interaction mediation. Although the examples used here are based on healthcare scenarios, the principles and abstractions described in the paper are more generally applicable.