Obesity and the Immune System

Obesity, the state of having excess body fat relative to lean body mass, alters the immune system of the individual who suffers from this condition. Obesity leads to the chronic active stimulation of the immune system from within the adipose or fat tissue, in the lymphoid tissue (where adipose tissue and lymph nodes sit in proximity to one another), and through distant messaging systems. Adipocytes (fat cells) and the immune system are in frequent communication with each other, leading to inflammation and modulation of signaling that has ultimately been implicated in the pathogenesis of diseases related to obesity.

An effective immune response to injury is critical to species survival. Typically, this response is swift and strong, dealing quickly with insults. However, with obesity, the hypothesized insult is stress on the organism from excess fatty acid accumulation and chronic oxygen insufficiency as the expanding fat mass outstrips its blood vessels (by which the tissue gets oxygen and nutrients). The result of this chronic inflammatory state is diseases such as heart disease, diabetes, asthma, high blood pressure, and osteoarthritis.

The immune system is an intricate network of cells that communicate through cell-to-cell and protein-to-cell messaging systems that affect clearance of insults to an organism. Classically, injury such as a cut leads to recruitment of fighter cells and repair mechanisms whose calling cards are protein messengers sent out by the injured cells like SOS signals. Researchers looking at adipose tissue (an organ unto itself) have found that adipose or fat tissue is sending out the very same calling cards, eliciting the help of the immune system. Unlike in the case of a cut, however, the fat mass often continues to grow and therefore continues to send out these signals, resulting in a chronic stimulation of the immune system.

Lymph nodes located throughout the body act as filters and sites of concentrated immune activity where lymphocytes (white blood cells that are primed or already trained to recognize foreign invaders such as bacteria and viruses) reside, and respond as the organism's blood stream comes into contact with infectious organisms. Surrounding the lymph nodes are layers of fat cells or adipocytes that collectively are called adipose tissue. Lymphoid tissue and adipose tissue reside together in the bone marrow (the adult site of blood cell production), in the apron-like sheet of fat covering the abdomen called the omentum, and in other concentrated sites such as the axilla (under the underarm), groin, popliteal fossa (behind the knee), and cervical regions (near the neck).

There appear to be many functions of this spatial relationship, including the provision of energy for the lymph tissue in the form of fatty acids from the adipocytes. For example, a foreign invader such as the breakdown product of bacteria (specifically called lipopolysaccharide which is the outer membrane of certain bacteria) is recognized by the lymph cells in the lymph node. They make cytokines (chemical signals indicating they are responding to an insult). The adipocytes intercept these help signals and provide fatty acids as energy for the battle. Inflammation (the multiplication of cells that respond in a specific fashion to the insult and the further manufacture of cytokines to kindle the fire and signal other tissues to send in backup supplies) results. In response to mild inflammation, the adipocytes are depleted of their fatty acids stores and decrease in size. [Page 544]However, there are signals to produce more fat cells and the number of layers of fat cells grows, increasing the number of fat cells surrounding the lymph node when there is chronic active inflammation.

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