Blood Groups

The ABO blood group system involves three antigens and two antibodies found in human blood. The three antigens are antigen A, antigen B and antigen O. The two antibodies are antibody A and antibody B. The antigens are present on the red blood cells and the antibodies in the serum. Regarding the antigen property of the blood all human beings can be classified into 4 groups, those with antigen A (group A), those with antigen B (group B), those with both antigen A and B (group AB) and those with neither antigen A or B but with antigen O that the immune system does not generate antibodies against (group O).

I and i antigens on red blood cells (RBCs) are subterminal portions of the same carbohydrate chains that carry ABH antigens. I and i are not allelic; rather, they differ in their branching structure. The i antigen, found predominantly on fetal and infant RBCs, is characterized by disaccharide units (Gal-GlcNAc) linked in a straight chain. During the first 2 years of life, many of these linear chains are modified into branched chains, resulting in the appearance of I antigens found on adult RBCs. I specificity develops at the expense of i antigens when the branched structures appear. Enhanced expression of i antigens is associated with leukemia and other blood related diseases.

P1PK blood group system consists of three glycosphingolipid antigens: Pk, P1 and NOR. The Pk phenotype is defined by reactivity to antibodies to anti-Pk. Pk antigen is a receptor for Shiga toxins produced by Shigella dysenteriae and some strains of Escherichia coli, which may cause hemolytic uremic syndrome (HUS). It is also a receptor for Streptococcus suis (zoonotic bacterium which can cause bacterial meningitis.