The correlation between malnutrition and vulnerability to infection has been well established (discussed previously here). While the immune dysfunction could be characterized it was not until the last 10-15 years that an exact mechanism began to resolve.
It all began with the discovery of a new hormone called leptin from an unexpected place, adipose tissue (fat cells). Leptin, a product of the obese (ob) gene, was discovered while looking for factors that regulate body fat. As a consequence of manipulating this gene in an attempt to regulate body fat, it was discovered that mice deficient in leptin had profound immune deficiencies.
The amount of leptin produced by adipose cells (fat cells) is directly proportional to the amount of fat in the cells. (The number of fat cells in adults does not change, their size just shrinks or swells.) Leptin levels drop as body fat decreases or during fasting. Once leptin levels fall below a threshold, the lack of leptin puts the mammalian body into a starvation response. Areas of leptin activity are signaled by the production of the leptin receptor (OBR gene). Tissues producing the leptin receptor include areas of the hypothalamus that regulate body weight, bone mass, and appetite; ovarian cells, beta-cells of the pancreas, endothelial cells, and bone marrow stem cells, macrophages, and lymphocytes (1). Leptin influences cellular function by directly interacting with peripheral tissues including immune cells in lymph nodes, bone marrow, pancreatic function and bone homeostasis, but also by triggering hormonal changes in the brain, specifically in the hypothalamus. Study of leptin levels has opened previously unsuspected linked between central nervous system control and the development of the immune system.
The Hormonal Trigger of the Starvation Response
As long as leptin levels stay within normal levels, all of the functions displayed above function normally. As the leptin levels drop, many of these functions are adversely effected. It is a wide-spread trigger for a starvation response. Why cripple the immune response during starvation? My best guess would be because of the huge energy expenditure required to keep the immune response running normally, especially in cellular proliferation.
When leptin levels drop too low, physiological dysfunction occurs in haematopoiesis (blood cell production), bone metabolism, glucose metabolism and angiogenesis (blood vessel production and maintenance) and immune suppression involving both the innate (non-specific) and adaptive immune system. During malnutrition, the size of the thymus gland shrinks with diminished T cell development. This may be one of the long-term consequences of childhood malnutrition. Children with congenitally low leptin levels have a higher mortality rate due to childhood infections (2).
With all the functions illustrated above, it’s not very surprising that malnutrition is the second most common cause of secondary immune suppression today (2). Alternatively, high leptin levels in obese people have also been linked with increased vulnerability to infection possibly through the development of leptin resistance due to prolonged exposure to excessively high levels of leptin (2). Food for thought considering that obesity was one of the only risk factors for a poor outcome during the recent H1N1 influenza pandemic. We have come to expect malnutrition induced immune suppression, but we may also have to consider over-nutrition induced immune suppression and/or autoimmunity as outcomes of immune dysregulation due to leptin resistance.
 La Cava, A., & Matarese, G. (2004). The weight of leptin in immunity Nature Reviews Immunology, 4 (5), 371-379 DOI: 10.1038/nri1350
 Procaccini C, Jirillo E, & Matarese G (2012). Leptin as an immunomodulator. Molecular aspects of medicine, 33 (1), 35-45 PMID: 22040697