Final answer:
The ABO group antibodies are naturally occurring. They are formed in response to exposure to A and B antigens at an early age.
Step-by-step explanation:
The ABO group antibodies are naturally occurring. The body is exposed to A and B antigens at an early age through foods and microbes. Individuals with type A blood have naturally-formed antibodies to the B antigen circulating in their blood plasma, referred to as anti-B antibodies. Similarly, individuals with type B blood have naturally-formed anti-A antibodies. Individuals with type AB blood, which has both antigens, do not have naturally-formed antibodies to either A or B antigens. People with type O blood lack both A and B antigens on their erythrocytes, but both anti-A and anti-B antibodies circulate in their blood plasma.