Metabolic and immune systems are very important for a living organism to survive. Since they play crucial roles in maintaining homeostasis, both systems work interdependently. In metabolic disorders like obesity and related diseases, lot of reports have been published about immune system impairment. In addition, in the case of malnutrition and calorie restriction, immune system is affected through several mechanisms. Different from the current point of view, I propose an unusual way of thinking about the relationship between immune and metabolic systems. The nutrient-based ecosystemic balance; the simple desires and rules like feeding or reproduction underlying very complex relations among species; the beneficial effects of caloric restriction; and the similarities between the pathways contributing obesity and infection made me took from a different perspective. Based on ecology and systems thinking, the present hypotheses suggest that pathologic organisms could be a kind of dietary source for lymphoid cells. If it is true, it will bring new and hopeful insights to all events related to metabolic and immune systems. At first, new mechanisms could be improved to defeat unbeatable organisms like Mycobacterium tuberculosis or HIV. The unphagocytosed organisms could be unknown tastes for our body's defenders. For example, the lymphoid cells could be classified as virovores, bacterivores, or fungivores like herbivores, and carnivores. Secondly, this basic relation could be used to solve huge problems caused by obesity related disorders like diabetes, because insulin would play more important roles for lymphoid cells while regulating the blood glucose level if the hypothesis is true. And the persistence of hyperglycemia would mainly effect lymphoid cells according to the current hypothesis. (c) 2007 Elsevier Ltd. All rights reserved.