Abstract: This invention is a model of the mammalian immune system, having at least two tiers, and its use in the prevention, screening, testing, treatment and research of a variety of diseases and conditions, all of which produce the enzyme, indoleamine 2,3-dioxygenase (IDO) or inspire the body to produce IDO in reaction to them. Unlike the current model of the immune system that has a single tier in the decision process of whether or not the immune system will act,(Same (Self)/Different (Non-Self), this model takes the process at least a step further. Before any attack can occur, all targets judged Different must be judged at least once more. This decision is Baby or Not a Baby. No attack can occur unless the target is judged Different/Not a Baby. IDO production is the determining factor for this judgment.