An infant does not usually produce ABO antibodies until 3 to 6 months of age. Antibodies found in the sera of newborns are almost always IgG, passively acquired from the mother. For this reason, serum testing of newborns to determine ABO grouping is not performed. Anti-A and anti-B titers are highest at ages 5–10 years, and then they gradually decrease. Thus, in elderly patients, ABO antibodies may be challenging to detect.
In patients with hypogammaglobulinemia, some leukemias, lymphomas, or patients taking immunosuppressive drugs, the expected antibodies may be weak or even absent, reflecting the low levels of gamma globulin in the patient's serum. These and other ABO typing discrepancies must be resolved before an actual ABO group can be determined.
