BLOOD BANKING

Overview

Immunohematology is the study of blood group antigens and antibodies, HLAs and antibodies, pretransfusion testing, detection of unexpected alloantibodies, immune hemolysis, autoantibodies, drugs, blood collection, blood components, cryopreservation, transfusion-transmitted infections, tissue/blood banking, transfusion practice, safety, quality assurance, and inventory management.

Antigen and immune-system basics

  • The immune system includes acquired (specific antibodies by plasma cells) and innate (non-specific barriers and leukocytes) components.

  • Antigens are substances that bind antibodies; immunogens elicit a response. RBCs, WBCs, and platelets carry antigens.

  • There are ~23 RBC antigen systems (>200 antigens total). RBC antigens are proteins, glycoproteins, or glycolipids.

  • HLAs (on leukocytes and tissues) are encoded by MHC genes on chromosome 6; class I (A,B,C) and class II (DR, DP, DQ) govern transplant compatibility and platelet/immune responses. Additional roles include paternity testing and donor matching.

  • Platelet antigens exist; platelet antibodies can be ABO, HLA, or platelet-specific. Diseases include neonatal alloimmune thrombocytopenia and posttransfusion purpura.

ABO and H antigen systems and secretor status

  • Landsteiner’s Rule: If an individual has an antigen, they usually do not have the corresponding antibody.

  • ABO antigens are glycolipids/glycoproteins expressed on RBCs, secretions, and other tissues; development begins in utero; full expression by 2–4 years of age.

  • ABO subgroups exist (A1, A2, others); A and B antibodies are IgM, typically not clinically significant except ABO antibodies; anti-A and anti-B can activate complement and cause agglutination.

  • The H antigen is the precursor for A and B; H gene (H) and rare amorph allele (hh, Bombay phenotype) determine expression. Bombay individuals lack H and have anti-H that can cause severe reactions; transfuse Bombay phenotype blood only.

  • Secretor status (Se vs se): Se individuals secrete ABH antigens in fluids; Se/Le determine secretions of A, B, H antigens in saliva, urine, etc. Substances in secretions depend on Le and Se genes; nonsecretors can have different secretion patterns.

Rh blood group systems

  • Rh system controlled by RHD (D antigen) and RHCE (C/c, E/e). Antigens are proteins; D is the most immunogenic.

  • D variants: Weak D, mosaic D, partial D. Weak D must be detected by IAT; partial D can trigger anti-D if exposed to normal D antigen.

  • Rh antibodies are IgG, generally do not activate complement; react best at 37°C; AHG phase recommended; can cause HDN and HTR; stronger reactivity with homozygous antigens (dosage).

  • Other Rh antigens (Cw, V, Ce, etc.) exist and can cause HTR/HDN; antibodies often IgG and may cross placenta.

  • RhIG (Rh immune globulin) prevents maternal anti-D formation after exposure to D+ fetal RBCs.

Other blood group systems

  • Kell (K, k), Kx; K is highly immunogenic; Kx is needed for Kell expression; McLeod phenotype (reduced Kell antigens, X-linked) and other high-/low-incidence antigens exist.

  • Duffy (Fy), Kidd (Jka, Jkb), Lutheran (Lua/Lub), Lewis (Lea/Leb), I, P, MNS (M/N; S/s; U), Diego, Cartwright, XG, Scianna, Dombrock, Colton, Chido/Rodgers, Gerbich, Cromer, Knops, Vel, JMH, Sid, and others. Many are high- or low-incidence antigens and have clinically significant antibodies (many IgG, AHG-detectable).

  • Clinical note: antibodies to high-incidence antigens require antigen-negative units; antibodies to low-incidence antigens can complicate finding compatible blood.

Blood bank reagents and methods

  • Reagent RBCs: known antigen profiles used to screen and identify antibodies.

  • Antisera: antibodies against RBC antigens; IgG/IgM specificity varies by system.

  • Antiglobulin reagents: detect IgG and/or complement in DAT/IAT; polyspecific (IgG+C3) or monospecific (IgG or C3).

  • Potentiators: enhance antigen–antibody reactions (e.g., LISS, PEG, albumin, enzymes).

  • Types of testing: ABO/Rh typing (forward/reverse), antibody screen, antibody identification (panel with multiple RBCs), crossmatch (IS, AHG, or electronic crossmatch).

  • Methods include tube tests, gel technology, microplate methods, solid-phase assays, IAT (sensitization and lattice formation), and elution/adsorption techniques to pinpoint antibodies.

Pretransfusion testing and crossmatching

  • Compatibility testing includes recipient identification, specimen handling, ABO/Rh typing, antibody screen, antibody identification, donor ABO/Rh confirmation, crossmatch, and antigen screening.

  • Crossmatch types: Immediate Spin (IS) crossmatch detects ABO incompatibility; Antiglobulin (AHG) crossmatch detects clinically significant antibodies; Electronic crossmatch relies on computer checks when history is negative.

  • Limitations: crossmatching cannot guarantee in vivo survival; it does not test for all transfusion-transmitted infections or WBC antigen compatibility.

  • Donor units are selected to minimize alloimmunization risk; antigen-negative units are used when necessary; consider leukoreduction to reduce febrile reactions and CMV transmission.

Antibody detection and identification; high- and low-frequency antibodies

  • Antibody screens detect clinically significant antibodies before transfusion; if positive, identification with a panel (10–20 cells) and dose analysis is required.

  • Panel interpretation uses autologous controls to distinguish autoantibodies from alloantibodies; the Rule of Three helps gauge accuracy of a new identification.

  • High-frequency antibodies react with most panel cells; low-frequency antibodies require testing with cells lacking specific antigens (
    “rare donor” testing).

  • Enhancing weak IgG antibodies may require different potentiators; adsorption/elution can identify or remove antibodies.

  • Cold antibodies (react at 4°C) may complicate testing; prewarming and adsorption help detect clinically significant antibodies.

Hemolytic diseases of the newborn (HDN)

  • HDN occurs when maternal IgG antibodies cross the placenta and lyse fetal RBCs; bilirubin accumulates, risking kernicterus.

  • Rh HDN is most severe; ABO HDN is common but usually milder; other IgG antibodies (Kell, Kidd, etc.) can cause HDN.

  • Prenatal management includes antibody titers, amniocentesis or fetal monitoring for severity, and RhIG prophylaxis for Rh-negative mothers.

  • Postnatal testing includes DAT on cord blood; prevention strategies include rosette and Kleihauer-Betke tests to estimate fetomaternal hemorrhage and guide RhIG dosing; exchange transfusion may be needed in severe cases.

Blood collection and donor selection

  • Donor screening includes demographics, health history, and exposure risks; donors must meet physiological criteria (weight, Hb, temp, BP, etc.).

  • Common deferrals include infectious risks and certain medications; autologous donations may be used in surgery; donor records retained with strict confidentiality.

  • Special donor programs include autologous donation, directed donations, and therapeutic/phlebotomy uses.

Blood components: preparation, storage, and shipment