Antibodies are naturally occurring when they are produced without any obvious immunizing stimulus such as pregnancy, transfusion or injection of blood. These antibodies are not present at birth and, in the case of anti-A and anti-B, start to appear in the serum of children with the appropriate ABO groups at about 3-6 months of age. ABO antibodies are probably produced in response to antigens of bacteria, viruses and other substances that are inhaled or ingested; many Gram-negative organisms have antigens that are structurally similar to the A and B antigens. Despite this probable antigenic stimulus, the term 'naturally occurring' is retained for these 'non-red cell-induced' antibodies. 'Immune' blood group antibodies are only produced after pregnancy or following transfusion or injection of blood or blood group substances.
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