Abstract

Biological boundaries are important because of what they reveal about the evolution of a lineage, the relationship between organisms of different lineages, the structure and function of particular subsystems of the organism, the interconnection between an organism and its environment, and a myriad of other important issues related to individuality, development, and evolution. Since there is no single unifying theory for all biological sciences, there are various possible theoretical characterizations of what counts as a biological boundary. Theoretical specificity is crucial for the full characterization of a boundary; to this end, it is useful to begin with general properties and then to compare these to those properties specific to a particular biological boundary. This dual heuristic approach—top-down and bottom-up—can then be repeated to increase the robustness of the boundary characterization. In what follows, I explore both the general structural and functional aspects of biological boundaries and those specific to a biological subsystem, which itself is critical to boundary formation, maintenance, and evolution—the vertebrate immune system. It is a remarkable and sobering regularity of nature that organisms remain whole because their parts are constantly being built up and broken down, and this is no less true of their boundaries. The vertebrate immune system is at the center of this process of biological boundary maintenance and breakdown