The ABO and Rh blood group systems are two of the most important systems for human blood transfusions and are classic examples of genetic inheritance in humans. They are determined by specific genes that code for antigens (proteins or carbohydrates) present on the surface of red blood cells.

I. ABO Blood Group System:

The ABO blood group system is governed by a single gene, the I gene (for isoagglutinogen), located on chromosome 9. This gene has three common alleles: I^A, I^B, and i.

• Alleles and Their Products:

  • I^A allele: Codes for the production of A antigen on the surface of red blood cells.
  • I^B allele: Codes for the production of B antigen on the surface of red blood cells.
  • i allele: Does not code for the production of either A or B antigen (it produces a precursor H antigen, but no A or B specific sugar).

• Inheritance Pattern: The I^A and I^B alleles are codominant with respect to each other, meaning that if both are present, both A and B antigens will be expressed. Both I^A and I^B are dominant over the i allele, which is recessive.

• Genotypes and Phenotypes (Blood Types):

  • Blood Type A: Individuals have A antigens on their red blood cells. Their possible genotypes are I^A I^A (homozygous) or I^A i (heterozygous).
  • Blood Type B: Individuals have B antigens on their red blood cells. Their possible genotypes are I^B I^B (homozygous) or I^B i (heterozygous).
  • Blood Type AB: Individuals have both A and B antigens on their red blood cells. Their genotype is I^A I^B. This demonstrates codominance.
  • Blood Type O: Individuals have neither A nor B antigens on their red blood cells. Their genotype is i i (homozygous recessive).

• Antibodies in Plasma: In addition to antigens on red blood cells, individuals naturally produce antibodies in their plasma against the antigens they lack. This is crucial for transfusion compatibility:

  • Type A blood has anti-B antibodies.
  • Type B blood has anti-A antibodies.
  • Type AB blood has neither anti-A nor anti-B antibodies (universal recipient plasma).
  • Type O blood has both anti-A and anti-B antibodies (universal donor cells).

II. Rh Blood Group System:

The Rh blood group system is primarily determined by the RHD gene, located on chromosome 1. While there are other Rh genes (RHCE), the D antigen, encoded by the RHD gene, is the most immunogenic and clinically significant. Individuals are classified as Rh-positive or Rh-negative based on the presence or absence of the D antigen.

• Alleles and Their Products:

  • D allele: Codes for the presence of the D antigen on red blood cells. This allele is dominant.
  • d allele: Represents the absence of the D antigen (often due to a deletion of the RHD gene or a non-functional gene). This allele is recessive.

• Inheritance Pattern: The D allele is dominant over the d allele. Therefore, only one copy of the D allele is needed for an individual to be Rh-positive.

• Genotypes and Phenotypes (Rh Types):

  • Rh-positive: Individuals have the D antigen on their red blood cells. Their possible genotypes are DD (homozygous dominant) or Dd (heterozygous).
  • Rh-negative: Individuals lack the D antigen on their red blood cells. Their genotype is dd (homozygous recessive).

• Antibodies in Plasma: Unlike the ABO system, individuals do not naturally produce anti-Rh (anti-D) antibodies. Anti-Rh antibodies are typically produced only after exposure to Rh-positive blood (e.g., through transfusion of Rh-positive blood into an Rh-negative person, or during pregnancy when an Rh-negative mother is exposed to Rh-positive fetal blood).

• Clinical Significance (Hemolytic Disease of the Newborn - HDN): The Rh system is critically important in pregnancy. If an Rh-negative mother carries an Rh-positive fetus, fetal red blood cells can enter the mother's bloodstream, especially during delivery. The mother's immune system may then produce anti-Rh antibodies. In subsequent pregnancies with another Rh-positive fetus, these maternal antibodies can cross the placenta, attack the fetal red blood cells, and cause severe anemia, jaundice, and even death in the fetus or newborn (erythroblastosis fetalis). This condition can be prevented by administering RhoGAM (Rh immune globulin) to Rh-negative mothers, which neutralizes any fetal Rh-positive cells in the mother's circulation before her immune system can produce antibodies.