An elementary model of homeostasis and immunity that generates symbiosis

The immune system was historically defined as a system that provides protection from pathogens. Numerous models have been developed to understand how immunity faces a complex world of microbes that includes pathogens and symbionts, as well as cells of our own self that may develop tumors. Based on the classical assumption that survival depends on internal homeostasis, we have developed a formal model of homeostasis for a host interacting with microbes and self. We propose that such a model must include two fundamental functions: a function that counters change (including tissue repair), and a function that counters the agent of change (such as "immunity" to microbes or self). We show that this elementary model is sufficient to generate symbiosis, and that symbiosis is an emergent property of the host-microbe relationship that does not require the microbe or the host to express "traits of symbiosis". We suggest that the conditions leading to symbiosis contribute to eukaryotic evolution and ontogeny. This model may be further applied to symbiotic interactions between organisms and non-microbial or non-cellular agents of change.