Gut bacteria and blood
About 80% of our immune system is located in the gut. This makes anatomical sense because: 1. the digestive system is where the majority of bacteria (and threats) reside and, 2. the immune system needs to be able to be trained by their presence and become able to recognize their friendliness or danger.
The immune system cells are in charge of recognizing all other cells and particles in the body in order to discriminate which are local and which are foreign and possibly threatening. The ability to recognize friend from foe is acquired by the immune system cells in the gut during the early stage of immune system maturation. In the gut, for the first time immune cells meet the bacteria and learn to accept the friendly symbiotic bacteria and to be prepared to fight the dangerous pathogenic ones.
Immune system cells recognize other cells by checking their surface structure. However, this system is very risky because sometimes the surface structure of both friendly and pathogenic bacteria is very similar to the surface structure of local cells. This similarity can cause mistakes by the immune system in recognizing dangerous cells (including bacteria) from locals, sometimes leading to either a friendly reaction towards invaders or an aggressive reaction towards local cells (as in autoimmune reactions).
Such danger relates for example to red blood cells: their surface has proteins that are very similar to the ones found on the surface of bacteria. Such proteins are the ones that identify differences between genetically-determined blood groups A, B and O. Such proteins on the surface of red blood cells are the reasons why a person can receive blood only from compatible groups. If the donor blood is of a different type, i.e. presenting red blood cells with different proteins on the surface, the immune system cells would recognize the blood cells as foreign cells, since very similar to that of bacteria, and attack them and coagulate them.
When a baby is born, his/her gut is not yet colonized by many bacteria and this blood type recognition problem does not exist. This is why babies could receive blood from any blood type. Once her/his gut flora is well developed, then the blood transfusion need to be better targeted. This observation speaks of a very tight connection between gut bacteria and blood type.
Blood type is one of the major factors causing the very high diversity and high individuality of the gut microbes composition. In fact, people of different blood types produce different types of mucus on their gut surface, which in turn feed different types of bacteria. Each composition of bacteria feed on different ingested food types and thus help the host (the human organism) digest certain foods better than others. This explains why eating food adapted to a certain blood type (The blood type diet, http://www.dadamo.com/txt/index.pl?3000) makes physiological sense.
Our individual microbiota gifted us in the past with an increased ability to absorb different nutrients that would be indigestible without such microbes. This gut microbiota has coevolved with us, receiving benefit from being hosted and fed and helping us in exchange with digesting new foods for us.
Harri Mäkivuokko, Sampo J Lahtinen, Pirjo Wacklin, Elina Tuovinen, Heli Tenkanen, Janne Nikkilä, Marika Björklund, Kari Aranko, Arthur C Ouwehand, and Jaana Mättö, 2012 Association between the ABO blood group and the human intestinal microbiota composition. BMC Microbiol. 12: 94.
Giulia Enders, 2015. Gut, the inside story of our body's most underrated organ.
Peter D'Adamo 2013, Gnomic Advice, at http://n-equals-one.com/blogs/2013/06/03/gnomic-advice/
Peter D’Adamo, 2011. Eterotypes and blood types, at http://n-equals-one.com/blogs/2011/04/22/enterotypes-and-blood-types2/