Antigen Expression - Antigen Development on Fetal Erythrocytes

         Most RBC antigens can be detected early in fetal development (A, B, and H antigens can be detected at 5–6 weeks' gestation), but not all are fully developed at birth. A, B, H, I, P1, Lu^a, Lu^b, Yt^a, Xg^a, Vel, Bg, Knops, and Dombrock antigen expression is considerably weaker on cord erythrocytes than on RBCs from adults. Le^a, sometimes Le^b, Ch/Rg, AnWj, and Sd^a, are not readily detectable, although 50 percent of cord samples type Le(a+) with more sensitive test methods. Full expression of A, B, H, I, and Lewis antigens usually is present by age 3 years, whereas full expression of P1 and Lutheran antigens may not occur until age 7 years.