Ii antigen system

[1] Enhanced expression of i antigen is associated with conditions involving stress hematopoiesis such as leukemia and sickle cell disease.

[7] Transient autoantibodies against I antigen are common, especially after infection by Mycoplasma pneumoniae, and are rarely significant except in cold agglutinin disease.

[8] The antibodies are usually IgM (kappa subtype), unlike transient autoantibodies which are generally IgG.

[1][8] Rarely, individuals have the i antigen on their red blood cells into adulthood, known as the adult i phenotype.

[1][3] These individuals have alloantibodies against the I antigen, though these are typically cold agglutinins and are unlikely to cause transfusion reactions.

[6] A similar blood group system with a developmental change resembling the Ii system (with human neonatal cells expressing i antigen and adult cells expressing I antigen) has been observed in most primates, including chimpanzees and monkeys.