Immunohaematology: ABO and Rh Blood Groups

• Immunohaematology involves studying antigen-antibody reactions related to blood group antigens
• It is crucial for safe blood transfusions and managing haemolytic diseases

Blood Group Systems

• Blood group systems are defined by the presence of specific antigens on the surface of red blood cells
• The ABO and Rh systems are clinically the most significant

ABO Blood Group System

• The ABO system includes four main blood groups: A, B, AB, and O
• These groups are determined by the presence or absence of A and B antigens
• Group A individuals have A antigens, group B have B antigens, group AB have both A and B antigens, and group O have neither
• Individuals naturally produce antibodies against the ABO antigens they lack
• Group A has anti-B antibodies, group B has anti-A antibodies, group O has both anti-A and anti-B antibodies, and group AB has neither

Rh Blood Group System

• The Rh system is complex, but the D antigen is the most important
• Individuals with the D antigen are Rh-positive (Rh+), while those without it are Rh-negative (Rh-)
• Rh incompatibility can cause haemolytic disease of the foetus and newborn (HDFN)
• Anti-D antibodies are usually only produced after exposure to Rh-positive blood

Antibody Screening and Identification

• Antibody screening detects unexpected antibodies in a patient's serum that could cause transfusion reactions or HDFN
• Panel cells with known antigen profiles are used to identify the specificity of the antibodies

Crossmatching

• Crossmatching involves mixing the recipient's serum with donor red blood cells to check for compatibility
• It aims to detect any antibodies in the recipient's serum that could react with the donor cells
• A compatible crossmatch indicates a low risk of a transfusion reaction

Direct Antiglobulin Test (DAT)

• The DAT detects antibodies or complement proteins attached to the surface of red blood cells
• It is used to diagnose autoimmune haemolytic anaemia (AIHA), HDFN, and transfusion reactions
• A positive DAT indicates that red blood cells are coated with antibodies or complement

Indirect Antiglobulin Test (IAT)

• The IAT detects antibodies in the serum that can bind to red blood cells
• It is used for antibody screening, antibody identification, and crossmatching
• The IAT involves incubating serum with red blood cells, followed by the addition of antiglobulin reagent

Blood Collection and Sample Preparation

• Proper blood collection is essential to avoid false results
• Samples should be collected in EDTA tubes for most immunohaematology tests
• Serum or plasma is used for antibody screening and identification
• Red blood cells are used for blood grouping and DAT

Quality Control

• Quality control procedures are necessary to ensure accurate and reliable results
• This includes regular testing of reagents and equipment
• Proper documentation and record-keeping are also essential

Transfusion Reactions

• Transfusion reactions can be classified as acute or delayed, and immune or non-immune
• Acute haemolytic transfusion reactions (AHTR) are often caused by ABO incompatibility
• Delayed haemolytic transfusion reactions (DHTR) are usually caused by Rh or other blood group antibodies
• Febrile non-haemolytic transfusion reactions (FNHTR) are caused by cytokines released from donor leukocytes
• Allergic reactions can range from mild urticaria to severe anaphylaxis

Haemolytic Disease of the Foetus and Newborn (HDFN)

• HDFN occurs when maternal antibodies cross the placenta and destroy foetal red blood cells
• Rh incompatibility is the most common cause, but ABO incompatibility can also occur
• Prevention involves administering anti-D immunoglobulin to Rh-negative mothers after delivery or sensitizing events
• Management includes monitoring the foetus for anaemia and performing exchange transfusions after birth if necessary

Autoimmune Haemolytic Anaemia (AIHA)

• AIHA is caused by autoantibodies that react with the individual's own red blood cells
• Warm AIHA is caused by IgG antibodies that react at body temperature
• Cold AIHA is caused by IgM antibodies that react at lower temperatures
• Treatment may include corticosteroids, immunosuppressants, or splenectomy

Blood Grouping Techniques

• Forward typing uses known antisera to detect antigens on red blood cells
• Reverse typing uses known red blood cells to detect antibodies in the serum
• Discrepancies between forward and reverse typing need to be resolved to ensure accurate blood grouping

Antiglobulin Reagent

• Antiglobulin reagent (Coombs reagent) contains antibodies against human IgG and/or complement components
• It is used to detect antibodies or complement attached to red blood cells in the DAT and IAT

Elution Techniques

• Elution techniques are used to remove antibodies from the surface of red blood cells
• The eluted antibodies can then be identified to determine their specificity
• Elution is useful in investigating positive DAT results and HDFN

Adsorption Techniques

• Adsorption techniques are used to remove specific antibodies from the serum
• Patient serum is incubated with red cells expressing the target antigen, allowing the antibody to bind
• Removal of antibodies helps to reveal coexisting antibodies or identify antibodies to high prevalence antigens

Cold Agglutinins

• Cold agglutinins are antibodies that react with red blood cells at cold temperatures
• High titres of cold agglutinins can cause agglutination of red blood cells in vitro and in vivo
• Pre-warming samples can help to avoid interference from cold agglutinins in blood grouping and crossmatching

Mixed Field Agglutination

• Mixed field agglutination occurs when only a proportion of red blood cells agglutinate
• This can be seen in situations where there are two cell populations, such as after a transfusion or bone marrow transplant
• It can also be seen in some weak antigen expressions.

Dosage Effect

• Dosage effect refers to the stronger reactions observed when an antibody reacts with red cells carrying a double dose of the antigen compared to a single dose
• Antibodies to some blood group systems, such as Rh, MNS, and Kidd, show a dosage effect
• Dosage effects means reactions may be weaker or absent if the antigen is only inherited on one chromosome

Enzymes in Immunohaematology

• Enzymes such as ficin, papain, and trypsin can modify red cell antigens
• Some antigens are enhanced by enzymes (Rh, Kidd, Lewis, and P), while others are destroyed (MNS, Duffy)
• Enzyme treatment can be useful in antibody identification

Lectins

• Lectins are plant extracts that have specific binding affinities for certain red cell antigens
• Anti-A1 lectin (Dolichos biflorus) is used to distinguish A1 from A2 subgroups
• Anti-H lectin (Ulex europaeus) reacts with the H antigen, which is present in varying amounts on different ABO blood groups.

Reporting Results

• Accurate and clear reporting of results is essential
• Reports should include patient details, test performed, results, and any relevant comments
• Any discrepancies or unexpected results should be investigated and documented

Safety Precautions

• Standard laboratory safety precautions should be followed
• This includes wearing personal protective equipment (PPE) and handling blood samples as potentially infectious
• Proper disposal of biohazardous waste is also essential