3. Digestion: immunity at its most powerful

By far our greatest exposure to foreign material is from what we eat Our successful immune defences depend entirely on this material being effectively broken down by a healthy digestion.

We have all heard that a successful organ transplant requires a donor with a close genetic match so that the new organ is not rejected. This is because the immune system is particularly alert to foreign proteins and will attack it as a threat to healthy body functions.

So what does the immune system do with all the foreign protein and other complex materials that we eat?

First and foremost it relies on our digestive system to make the food safe. Digestion renders down large molecules into their building blocks, like amino acids and simple sugars, which are not seen as a threat. A healthy digestive system does this very well and so is our main protection against immune challenges.

However there has to be a back up. No digestion can be 100% perfect and some material does escape breaking down even in health. So for this reason around 70% of our immune cells are located with a centimetre of the gut wall! These accumulations of white blood cells and the lymphatic vessels that concentrate them are collectively known as Gut Associated Lymphatic Tissue (GALT).

We are familiar with tonsils and the appendix as lymphatic concentrations at the upper and lower end of the gut. In the lower small intestine and large bowel there are more diffuse concentrations called ‘Peyers patches’ that come in almost direct contact with the gut contents.

As expected the gut is where large quantities of antibodies or ‘immunoglobulins’ (Igs – see Article 5) are produced. The most common type around the body is classified as IgG. The gut also expresses surface immunoglobulins IgA and IgD, and lower down the gut there are concentrations of IgM assoicated with the Peyer’s patches.