A person's blood type is defined by the types of sugars they express on their red blood cells, with antibodies being made to non-self sugars and excluding blood cells expressing these sugars (the basis of blood transfusion reactions). These same sugars can be expressed by commensal bacteria in our gut lumens. The investigators hypothesize that a person's antibodies to non-self sugars also attach to and exclude non-self sugar-expressing bacteria in our guts. Supporting our hypothesis, a person's blood type correlates with the overall composition of their gut flora. Because these sugars are also used by certain pathogens to initiate infection, this information could directly inform our understanding of host susceptibility to various infectious organisms.