Medical Laboratory Science

Blood banking, also called immunohematology, involves the study of blood group antigens and antibodies, compatibility testing, and transfusion medicine. The field requires understanding of antigen-antibody reactions, inheritance patterns, and clinical applications in transfusion therapy.

Antigens are molecules capable of stimulating an immune response. Red blood cell antigens are glycoproteins or glycolipids embedded in the cell membrane. These antigens are inherited and remain constant throughout life. Antibodies are immunoglobulins produced by plasma cells in response to foreign antigens. In blood banking, we primarily deal with IgG and IgM antibodies.

IgM antibodies are large pentameric molecules that cannot cross the placenta. They react optimally at room temperature or below and directly agglutinate red cells in saline. IgM antibodies fix complement efficiently. IgG antibodies are smaller monomeric molecules that cross the placenta and can cause hemolytic disease of the fetus and newborn. They react optimally at 37°C and require enhancement media or antiglobulin testing for detection.

Complement is a group of serum proteins that participate in immune reactions. The classical pathway is activated by antigen-antibody complexes. Complement activation can lead to intravascular hemolysis through the membrane attack complex. C3b and C3d fragments deposited on red cells are detected by polyspecific antiglobulin reagents.

The ABO system is the most clinically significant blood group system. ABO antigens are carbohydrate structures synthesized by glycosyltransferase enzymes. The H gene encodes fucosyltransferase that produces the H antigen, which serves as the precursor for A and B antigens.

The A gene encodes N-acetylgalactosaminyltransferase that adds N-acetylgalactosamine to the H antigen. The B gene encodes galactosyltransferase that adds galactose to the H antigen. The O gene is an amorph that produces no functional transferase, leaving the H antigen unmodified.

Group A individuals have A antigens and anti-B antibodies. Group B individuals have B antigens and anti-A antibodies. Group AB individuals have both A and B antigens and no ABO antibodies. Group O individuals have neither A nor B antigens but possess both anti-A and anti-B antibodies.

ABO antibodies are naturally occurring, developing in the first few months of life through exposure to environmental antigens similar to A and B substances. These antibodies are predominantly IgM but may have IgG components, especially in group O individuals.

ABO subgroups result from quantitative or qualitative differences in antigen expression. A1 is the most common A subgroup with approximately one million A antigen sites per red cell. A2 has fewer antigen sites and may have anti-A1 in the serum. Weak A subgroups include A3, Ax, Aend, Am, and Ael, with progressively weaker antigen expression. Similar subgroups exist for group B.