Introduction to Immunohematology and Genetics

Study Notes

Immunohematology is a specialized branch of medical science focused on transfusion therapy, blood components, and their derivatives.
It entails the serologic, genetic, biochemical, and molecular examination of blood-borne antigens.
Immunohematologists conduct various serologic and molecular tests to facilitate diagnoses in transfusion, pregnancy, and organ transplants.
Research in immunohematology has greatly advanced the understanding of human genetics, immunology, and applications in various scientific fields.

Historical fascination with blood dates back to ancient practices in Egypt.
William Harvey’s discovery of blood circulation in 1616 marked the beginning of systematic blood transfusion study.
Richard Lower performed the first successful animal-to-animal transfusion in 1665, supporting exsanguinated dogs.
Jean Baptiste Denys's lamb blood transfusions in 1667 highlighted species incompatibility challenges, leading to a temporary ban on transfusions until 1818.
Successful human blood transfusions by James Blundell in the 19th century improved hemorage management during childbirth.
Karl Landsteiner’s discovery of ABO blood groups in 1900 established the importance of specific red cell antigens in transfusion compatibility.

Blood groups are inherited traits determined by genes located on chromosomes, containing a combination of dominant, recessive, and codominant alleles.
Each human cell contains 46 chromosomes organized in pairs, housing genes that dictate various traits, including blood type.
Inheritance details:
- Homozygous individuals inherit identical alleles (e.g., A/A).
- Heterozygous individuals inherit different alleles (e.g., A/O).

Phenotype refers to the observable traits, while genotype indicates the genetic constitution.
Hemagglutination tests with antisera help ascertain the phenotype of blood types.
For example, a person may present type O blood without agglutination with anti-A or anti-B antisera.

Punnett squares predict offspring genetic probabilities from maternal and paternal allele combinations.
Genes are located at specific loci on chromosomes; alleles represent variations of those genes.
Genes act as basic inheritance units, affecting the expression of antigens through recessive or dominant patterns.

Inheritance principles include independent segregation of alleles and independent assortment of traits from different chromosomes.
Exceptions to these principles include genetic linkage (genes inherited together) and crossing over (exchange of genetic material between chromosomes).

Most blood group genes are located on autosomes, with exceptions like the Xg system.
Heterozygous individuals display different phenotypes due to varying allele combinations.
Dosage effect occurs when homozygous individuals exhibit stronger antigen expression compared to heterozygotes.

Population genetics assesses the frequency of genotypes or phenotypes within a population using two primary formulas:
- Hardy-Weinberg formula calculates gene frequencies.
- Phenotype calculations determine the probability of compatibility for blood transfusions.

Inheritance of A and B genes influences the presence of specific antigens on RBCs.
The H gene encodes for the precursor substance (H antigen) involved in forming A and B antigens.
Individuals without the H gene cannot produce A or B antigens, leading to the Bombay (Oh) blood group.

Secretors possess water-soluble ABH antigens in bodily fluids; non-secretors lack this trait.
Se gene is responsible for antigen secretion, inherited independently of ABO and H genes.
Individuals homozygous for se do not secrete ABH antigens, irrespective of main blood group genes.