4. ABO and Rh testing
Immunogen: Also known as an “antigen.” A substance that reacts with an antibody and is usually foreign to the body. Not all antigens trigger an immune response; all immunogens are antigens, but not all antigens are immunogens. Most immunogens are proteins.
Immunoglobulin: Known as an “antibody,” it is a protein molecule produced by plasma cells derived from B lymphocytes. They are generated in response to foreign immunogens. The five classes of antibodies are IgM, IgG, IgA, IgD, and IgE, with IgM and IgG being the most clinically significant in transfusion medicine. IgM is typically clinically insignificant, while IgG can cause red blood cell destruction at body temperature.
Isoagglutinin: Antibodies, particularly referred to as ABO antibodies, which are primarily IgM and are usually naturally occurring.
Naturally Occurring Antibody: Antibodies present without known exposure to the corresponding antigen, generally arising from environmental exposures (e.g., bacteria or food).
Immune Antibody: Also known as alloantibodies, formed after exposure to foreign antigens via pregnancy or transfusion. Usually IgG, they optimally react at 37°C and can cause hemolytic diseases and reactions.
Isotype Switching:
Process of B cells changing antibody class without changing antigen specificity, most common in transfusion contexts is from IgM to IgG.
Discovery of the ABO Blood Group System:
Discovered by Karl Landsteiner in 1900 and further researched in 1901. Landsteiner mixed blood samples with plasma and recorded results, noting the agglutination of red blood cells (RBCs).
Individuals may have A antigen, B antigen, both A and B antigens, or none. Plasma contains antibodies against the A and B antigens absent from one’s own RBC.
Blood groups A, B, AB, and O were identified, with AB being discovered in 1902 by Landsteiner's students.
Universal Donor: Group O is considered a universal donor due to the absence of A and B antigens, meaning it does not elicit an immune response when transfused. Clinically, O negative (O-) is the true universal donor as it lacks the Rh (D) antigen.
Universal Recipient: Group AB is seen as a universal recipient, able to receive any ABO type without an immune reaction because it possesses both antigens and no corresponding antibodies.
ABO Blood Group Systems and Their Antibodies:
Clinical Significance: Antibodies in the ABO group can lead to severe transfusion reactions. IgM is clinically significant in the ABO system as it can cause rapid hemolysis.
Forward Typing and Reverse Typing: Testing methods for identifying blood types consist of detecting antigens present on patient RBCs and the antibodies in the plasma, respectively.
Naturally Occurring Antibodies:
Develop without prior exposure to corresponding antigens, often initiated by environmental factors, leading to production of antibodies that may cross-react with RBC antigens.
Example: Infants born with maternal antibodies develop naturally occurring IgM antibodies post-birth through exposure to food and bacteria.
Immune Antibodies:
Formed following exposure to foreign antigens through various means and classified primarily as IgG.
Example: An Rh-negative individual's exposure to D antigen leading to production of anti-D antibody.
Antibody Screen Test:
Known as the indirect antiglobulin test (IAT) or Coombs test, it detects unexpected, clinically significant antibodies in patient plasma against non-ABO antigens such as Kell or Duffy. It is vital to perform this test prior to transfusion to prevent hemolytic transfusion reactions (HTRs).
Distinction between Genotype and Phenotype:
Genotype: The genetic constitution regarding blood type.
Phenotype: The observable characteristics, often determined through testing results, such as the presence of A, B antigens, or O type.
Immunologic Concepts Demonstrated by ABO Typing:
Specificity: anti-A reacts only with A antigen and anti-B reacts only with B antigen.
Agglutination: The visible clumping of cells in an antigen-antibody reaction.
Landsteiner’s Rule: If the antigen is present, the antibody is absent, and vice versa.
Serological Tests and Blood Typing
Forward Typing: Blood typing to detect antigens on patient's RBCs, testing against known antisera (Anti-A, Anti-B).
Reverse Typing: Testing plasma against commercial reagent cells to detect antibodies.
IgM antibodies are clinically significant, particularly in ABO system due to their potential for rapid intravascular hemolysis.
Grading Serological Reactions
Reaction Scale: Graded from 0 (no reaction) to 4+ (significantly positive reactions).
Resolving Discrepancies in ABO Typing:
Discrepancies arise due to unexpected test results, often due to technical or clerical issues.
Types of discrepancies include forward discrepancies (weak or absent reactions) or reverse discrepancies (unexpected reactions).
Common categories of discrepancies:
Category I: Unexpected reverse reactions due to weakly reacting or missing antibodies, commonly seen in certain patient populations such as newborns or patients with hypogammaglobulinemia.
Category II: Unexpected forward type reactions due to weak or missing antigens, possibly due to conditions like leukemia.
Category III: Protein abnormalities causing rouleaux formation.
Category IV: Cold autoantibodies leading to discrepancies between forward and reverse testing.
General rules for resolving discrepancies involve checking for technical errors, retesting, and considering patient history.
Specific techniques for resolving issues include incubating cells at various temperatures or using alternative testing methods based on suspected reactions.
Important Clinical Implications
Transfusion Reactions: Administering incompatible blood triggers severe immune responses, including hemolysis, shock, and even death. Patient monitoring is critical during transfusions.
Testing Requirements: Antibody screening is mandatory before transfusion; discrepancies must be resolved to ensure patient safety.
Understanding Antigens and Antibodies: Recognition of blood type compatibility is crucial for preventing hemolytic transfusion reactions and ensuring patient welfare during transfusions.
Summary of ABO System Properties
Antigen presence on RBCs is fundamental for determining blood type.
Antibodies in plasma can cause severe reactions if incompatible blood is transfused.
IgM and IgG serve distinct roles in transfusion medicine, with IgG being particularly significant for its capability to induce adverse immune responses at body temperature.