Immune Hemolysis Flashcards
Immune hemolytic anemia
Primarily extravascular hemolysis
• Ab-coated RBC ingested by macrophages
• Exception: complement-mediated intravascular hemolysis with IgM isoantibodies in ABO transfusion reaction (rarely with other IgM or IgG Abs)
Spleen = more efficient in clearing RBC coated with IgG Abs
Liver = more efficient in clearing cells sensitized by IgM autoantibodies
Autoimmune hemolytic anemia
Types:
• Primary or secondary
• Warm (IgG) or cold (IgM) antibody
Causes:
• Many cases = idiopathic
• Secondary causes: malignancies (especially lymphomas), certain infections, rheumatologic disorders, drugs
Warm Antibodies
o IgG mediated
o Optimum activity at 25-37 °C
o 70% of cases
o Extravascular»_space; intravascular hemolysis
DAT = IgG positive +/- C3
• Complement usually NOT activated
• Can be partially activated through C3
Most hemolysis in spleen
Clinical signs:
• Anemia = weakness, malaise, dyspnea on exertion, lightheadedness
• If have CVD = CHF or angina pectoris
Peripheral blood = often see spherocytes and shift cells
• RBCs have Ab’s attached = spleen removes bits of membrane → spheres
Treatment:
Corticosteroids (prednisone)
• Interfere with Fc receptor function
• Decrease autoantibody production
Splenectomy for relapsed disease or steroid failure
Additional immunosuppressive treatment (rituximab) for refractory disease
Transfusion if needed (may be hard to cross match blood)
Cold Antibodies
o IgM mediated
o Optimum activity at 0-4 °C or room temp
o 30% of cases
Causes: Idiopathic Also associated: • Following infection: mycoplasma pneumonia; infectious mononucleosis • Malignant lymphomas
Most common targets = I or i antigens on RBCs
• Most idiopathic and those associated with mycoplasma infections = anti-I
• With infectious mono and lymphomas = anti-i (chemical precursor of I antigen found on fetal and infant RBCs)
• Result = rare to have hemolysis from anti-i (not well expressed in adults)
Hemolysis: commonly extravascular, but may be intravascular if complete complement activation
• When extravascular = occurs mostly in liver (hepatic macrophages have C3 receptors)
DAT: positive for C3 and negative for IgG
Most cold agglutinins = clinically benign
• Most antibodies = active only at low temperatures (28°C)
• Will not be a problem if Ab only binds RBC at colder temperatures
Labs: see clumped RBCs
Clinical signs: Cold Agglutinin Syndrome
• Vasculopathy, blue fingers (Raynaud’s Phenomenon)
• Pain, paresthesias, cyanosis
• May cause cold-induced RBC vascular sludging (acrocyanosis)
Treatment:
• Corticosteroids = less effective than warm Ab hemolysis
• Splenectomy = often ineffective because hemolysis occurring in liver
• Alternative immunosuppression (anti-CD20 monoclonal Ab like Rituximab)
• Avoid cold exposure
Discuss Antiglobulin tests
Overall: Tests rely on agglutination of RBCs by antibodies
o IgM Ab’s = directly agglutinate (useful in ABO typing)
o IgG Ab’s = need help to agglutinate
• Add albumin to reduce surface charge
• Add 2nd Ab to link RBCs together
Direct Antiglobulin Test
o Detects IgG Abs (or complement C3) on patient’s RBCs
o Useful in evaluating hemolytic anemia
False negatives
• Coombs reagent cannot detect <100-500 molecules of Ab or C3 per RBC
• If lower number of Ab or C3 is producing hemolysis → false negative DAT
False positives
• Can occur if coexisting autoimmune disease or drug therapy
• Result = Positive DAT is NOT proof of immune hemolysis
Indirect Antiglobulin test
o Detects RBC IgG Ab’s in patient’s serum
o What blood bank uses to screen for “unexpected” Ab’s
Extra- vs. Intra-vascular hemolysis
Extravascular hemolysis
o Extravascular autoimmune hemolysis more common than intravascular autoimmune disease
o Primary cause: IgG autoantibodies
• Occasionally from IgM antibodies with incomplete complement activation
o Spleen = efficiently filters IgG-coated RBCs
• Has macrophages with receptors for Fc part of IgG
• Some also have receptors for C3b
• Synergistic action in binding IgG and C3b
o Liver = predominantly filters IgM-coated RBCs
• Has more macrophages with receptors for C3b
• Note: if large numbers of IgG molecules on RBC, liver becomes major site of hemolysis
• Proportional to blood flow: liver receives 30% CO; spleen only 5% CO
Intravascular hemolysis
o Requires fixation and complete complement activation
o IgM isoantibodies (ABO incompatibility) = usually cause intravascular hemolysis
o IgM autoantibodies = usually do NOT
• Instead = bind complement only through C3b = cleared by liver
Intra- vs. Extra- vascular hemolysis = determinants of complement activation:
IgM autoantibodies
• More likely to activate complement to C9
• IgG = able to do so but requires 2 molecules of IgG
Antibody subclass
• IgG4 = NOT activate complement
• IgG1 and IgG3 = strong activators
• IgG2 = weak activator
Proximity and number of antigen binding sites on RBC membrane
• Many more ABO antigens on surface → intravascular hemolysis more likely in ABO mismatches
• Fewer Rh (D) antigens → extravascular hemolysis more likely in mismatches
Describe three mechanisms of drug-induced hemolysis.
Drug binds to RBC membrane= forms new epitope that Ab can bind to → antibody binds to drug-membrane complex
o Drug increases affinity of pre-formed Ab to RBC membrane
• Reason why some drugs cause hemolysis with first exposure
o Most common mechanism
o Usually IgG antibodies
o Usually extravascular hemolysis
o Complete recovery after stopping drug
o Beta-lactam antibiotics most common cause (ex: penicillns, cephalosporins)
Drug binds to antibody in plasma; immune complex deposits on RBC membrane
o Often IgM antibodies w/complement fixation
o Often intravascular hemolysis
o DAT = C3 positive
o Quinidine (anti-arrhythmic) is prototype drug
Drug stimulates immune system to make autoantibodies vs RBC
o Rare
o Induced production of IgG antibodies against Rh antigen
o Extravascular hemolysis
o Alpha-methyldopa (anti-hypertensive) is prototype drug
NOTE: some drugs may cause positive DAT but NO hemolysis
o Instead = cause membrane modification → non-specific reaction to test
Hemolytic Disease of the newborn: list the common causes
o Hereditary elliptocytosis o Hereditary spherocytosis o G6PD deficiency o Alpha thalassemia o Maternal IgG antibodies to fetal RBC • Cause 98% of cases
Hemolytic Disease of the newborn: features
Maternal IgG crosses placenta and attaches to fetal RBC antigen
• Extravascular hemolysis in spleen → anemia, unconjugated bilirubin
• Unconjugated bilirubin not a problem because cleared by placenta
Severe hemolysis → extramedullary hematopoiesis, appearance of RBC precursors in bloodstream (erythroblastosis fetalis)
Liver can become obstructed and injured → hepatosplenomegaly, edema, ascitesd = hydrops fetalis → intrauterine or neonatal death
Early postnatal period:
• Unconjugated bilirubin → jaundice (immature liver can’t conjugate bilirubin)
• Can cause brain damage (kernicterus from damage to basal ganglia)
Hemolytic Disease of the newborn: types
Most common cause = ABO incompatibility (O mother, non-O fetus)
• Types A, B, and AB make IgM antibodies (can’t cross placenta)
• Type O makes IgG anti-A and anti-B abs
Disease is usually mild or asymptomatic because:
• Most people secrete ABH antigens → bind with Ab = protects RBC
• ABO antigens less well-developed at birth
IgG Abs = naturally occurring = no primary immunization needed
• HDN may occur with 1st pregnancy
More serious cause = Incompatibility for other RBC antigens (Ex: Rh, Kell, etc.)
• Rh- mother, Rh+ fetus = IgG anti-D crosses placenta
• More likely to cause severe hemolysis
Rh incompatibility most severe
• Less common now due to transfusion practices
• Administration of RhoGam (Rh immunoglobulin)
• Requires prior sensitization (not in 1st pregnancy unless mother previously transfused)
Maternofetal ABO incompatibility makes sensitization much less likely
• Protective effect
• Fetal cells immediately destroyed = no chance of sensitization
• High likelihood of recurrence → prevention is important!
Hemolytic Disease of the newborn: treatment
o Exchange transfusion (O neg RBC) for severe anemia
o Phototherapy for hyperbilirubinemia
o Intrauterine transfusion or early delivery in selected cases
Hemolytic Disease of the newborn: prevention
Never give Rh+ blood to Rh- woman of childbearing age
Administer RhoGam ~28 weeks to Rh- mothers; again after birthing an Rh+ child
Also give after abortion, miscarriage, amniocentesis, etc. (any event that might introduce fetal blood to maternal circulation)
• Prevents immunization in 90% of women at risk
• Mechanism uncertain (antigen blockade?)
• Possible additional benefit from administering another dose during third trimester
