Can an engineer fix an immune system?--Rethinking theoretical biology.

In an instant classic paper (Lazebnik, in Cancer Cell 2(3); 2002: 179-182) biologist Yuri Lazebnik deplores the poor effectiveness of the approach adopted by biologists to understand and "fix" biological systems. Lazebnik suggests that to remedy this state of things biologist should take inspiration from the approach used by engineers to design, understand, and troubleshoot technological systems. In the present paper I substantiate Lazebnik's analysis by concretely showing how to apply the engineering approach to biological problems. I use an actual example of electronic circuit troubleshooting to ground the thesis that, in engineering, the crucial phases of any non-trivial troubleshooting process are aimed at generating a mechanistic explanation of the functioning of the system, which makes extensive recourse to problem-driven qualitative reasoning possibly based on cognitive artifacts applied to systems that are known to have been designed for function. To show how to translate these findings into biological practice I consider a concrete example of biological model building and "troubleshooting", aimed at the identification of a "fix" for the human immune system in presence of progressing cancer, autoimmune disease, and transplant rejection. The result is a novel immune system model--the danger model with regulatory cells--and new, original hypotheses concerning the development, prophylaxis, and therapy of these unwanted biological processes. Based on the manifest efficacy of the proposed approach, I suggest a refocusing of the activity of theoretical biologists along the engineering-inspired lines illustrated in the paper.