The gastrointestinal barrier wall is approximately 400 m2 in surface area. It is a highly selective semi-permeable barrier between the inside of your body and the tube of the gut connected to the outside world. The purpose of this gut wall is to filter nutrients through the wall while keeping everything else out. This selective barrier is composed of epithelial cells and is always covered with a mucous defense layer through immune servailence. About 85% of the epithelial cells regulate nutrient absorption and are called enterocytes. The other 15% are sampling "microfold" cells (antigen presenting cells, endothelial cells, neurons, inflammatory cells, mast cells, smooth muscle cells and fibroblasts) that are involved with immune surveillance and defence.
The sides and bottoms of the enterocyte cells are smooth, while the tops facing into the gut are shaped into thousands of fingerlike projections (called microvili) which increase their surface area to improve uptake and transport of nutrition (Figure 1.). The main method by which nutrients are selectively absorbed into the body is to pass through these epithelial cells. The enterocyte cells bond to their neighbours by adhesive-protein-chemicals. These cells also have the ability to increase nutrient absorption when necessary in a less selective capacity by opening and closing the areas between the neighbouring cells which takes about 20 minutes in healthy people. These areas can act as non-selective gateways and are called "gap junctions" or "tight junctions".
If a tight junction becomes damaged, or an enterocyte cell’s ability to control the gateway is reduced, then the gap-junctions become "leaky" and this means that unwanted chemicals and microorganisms can cross the gut barrier into the body proper without immune control. When they get inside the body the immune system responds. A sign of this is lymphodema swelling of the abdominal region—the "beer gut" syndrome.