Antigen Expression - Variation in Antigen Expression

        RBCs from individuals who are homozygous for an allele typically have a greater number of antigen sites than do individuals who are heterozygous. Consequently, their RBCs can react more strongly with antibody. This difference in expression and antigen–antibody reactivity because of zygosity is known as dosage. For example, RBCs from a homozygous MM individual carry a double dose of M antigen and react more strongly with anti-M than do RBCs from a MN heterozygous individual carrying only a single dose of M. Antithetical antigens C/c, E/e, M/N, S/s, and Jk^a/Jk^b commonly show dosage. Dosage is less obvious with D, K/k, and Lu^a/Lu^b antigens. It typically is more apparent within a family than between families. Dosage within the Duffy system also may not be serologically obvious because Fy(a+b–) or Fy(a–b+) phenotypes are seen in either homozygous (Fy^aFy^a or Fy^bFy^b) or heterozygous (Fy^aFy or Fy^bFy) individuals.

Rh Blood Group System

        The Rh (not Rhesus) system is the second most important blood group system in transfusion medicine because antigen-positive RBCs frequently immunize antigen-negative individuals through transfusion and pregnancy.

       Inheritance of Rh antigens is determined by a complex of two closely linked genes: one encodes the protein carrying D antigen (RhD); the other encodes the protein carrying C or c and E or e antigens (RhCE). RBCs from Rh-positive people have both RhD and RhCE, whereas Rh-negative RBCs have only RHCE. In the Rh system, eight common antigen combinations or haplotypes are possible: Dce (R₀, Rh₀), DCe (R₁, Rh₁), DcE (R_2, Rh₂), DCE (R_Z, Rh_z), ce (r, rh), Ce (r', hr'), cE (r″, hr″), and CE (ry, rh″). The letter "d" is commonly used to designate the lack of D, but there is no d antigen or anti-d.