The Role of the Human Immune System in Obesity

Human immunity has developed, over evolutionary time, into a complex system that impacts a great deal of human biology.  It is for this reason that it often a key player in the cause of many ailments.  It has recently been implicated in human obesity, insulin resistance and type 2 diabetes – these three conditions constitute the so-called metabolic syndrome.  Immune responses are potent reactions to bodily insults and, therefore, must be tightly regulated.  These responses are activated and suppressed by a large family of small regulatory proteins referred to as interleukins.

The white blood cells (WBC) that are found circulating in human blood are, in fact, comprised of a complex mixture of cell types that all perform unique immunological roles.  Eosinophils are a particular WBC type that is classically involved in combating helminth (parasitic worm) infections and allergies.  In addition, macrophages are a subtype of WBC that is characterized by its large size and ability to respond to a wide range of human infections. 

Extensive studies of obesity using animal models led by Dr. Davina Wu at the Howard Hughes Medical Institute at the University of California in San Francisco have implicated Eosinophils and a particular subtype of macrophage referred to as alternatively activated macrophages (AAMs) in the maintenance of glucose balance in fat (Adipose) tissue.  It appears that the role of eosinophils in this process involves the secretion of a type of Interleukin called Interleukin-4 (IL-4).  When eosinophils are not present in adipose tissue, the level of glucose is no longer subject to regulation.

Doctor Wu and colleagues, working with the mouse model, have shown that the macrophages resident in the adipose tissue of obese subjects are actively involved in an inflammatory response while the macrophages in the adipose tissue of lean mice are not.

This study reinforces the established relationship between metabolism and immunity.  Furthermore, these findings support the connection between obesity and the enrichment of those genes involved in inflammatory and immune responses in humans – a strong indication of the health dangers posed by obesity.