The power of modern computers allows the modeling and simulation of complex biological systems. The last decade has seen the emergence of a growing number of simulations of the immune system. In this article, Franco Celada and Philip Selden present a model that, they suggest, is rich enough to allow computer experiments to be used as practical adjuncts to the usual biological experiments, at a saving of cost and time.

According to the logician Peirce 1, abduction-the scientific (but also Holmesian) hypothesis-is the most fruitful tool of the investigatory mind. It does not descend from principles or spring directly from empirical observation. It is the product of an educated imagination and establishes surprising connections between realities, but requires independent experimental proof. Often the very experiments are the bottleneck. They may not be readily performable for a number of reasons. Hypotheses that tend to undermine traditional views are the most stimulating but may also be the most difficult to test and the least likely to survive. Experimenters are known to hesitate before engaging in high risk experiments: this is only natural but leaves many wild and perhaps important experiments undone. One reason for hesitation is that biological experiments, both in vivo and in vitro, are time consuming and expensive. Here, a new class of experiment is proposed, which we dub in machina, that can be performed in a computer. These experiments will have two advantages. First, they will be relatively cheap and easy to do. Second, in contrast to the usual biological experiments, everything about the experiment can be measured in all its detail.