It is possible that since anti-A and anti-B develop so predictably, without a recognizable immunizing event, that they are naturally occurring. However, some exposure to antigen must take place for antibodies to be produced. ABO antibodies are generally not present in the serum of newborns. It is postulated that production of the antibodies occurs as a result of exposure to environmental and internal (digestive tract) bacteria, which have been shown to contain carbohydrate structures (antigens). Antibody production occurs after exposure to these carbohydrate antigens, in accordance with the person's genetically predetermined ABO group.