Introduction

Classification of AIHA

Warm-type antibody AIHA

Cold-type antibody AIHA

Other AIHAs

Biochemical Research

Future Directions

References

 

Introduction

Autoimmune hemolytic anemia (AIHA) refers to the potentially lethal, diverse disorder in which individuals produce antibodies directed against one of their own erythrocyte membrane antigens, resulting in the decreased survival of erythrocytes.  Hayem is credited with the production of the first recognizable description of acquired hemolytic anemia in 1898 (Dacie, 1992).  However, it was Widal, Abramni and Brulé who accurately described AIHA in a series of papers after 1908.   AIHA is a rare disorder.  In Letman’s studies in 1957, it was found that warm AIHA had a frequency of occurrence of 1 in 75,000 per annum in the Danish population.  In an isolated study of Pirofsky (1969), it was reported that an average of 1 in 80,000 people suffered from AIHA per annum in the Scandinavian population in Oregon.  Although AIHA is widely studied clinically, its etiology is not fully known.   

  back to top

Classification of AIHA

Depending on the site of erythrocyte clearance, AIHA can cause extravascular or intravascular hemolysis.  Intravascular hemolysis is characterized by complement activation and the subsequent destruction of erythrocyte membrane by the membrane attack complex.  It can proceed via the classic or the alternative pathway of complement activation.  In the classic pathway, binding of the complement-fixing antibody to the erythrocyte membrane antigen is followed by the binding of C1 to the Fc portion of the auto-antibody.  Subsequently, complement cascade is initiated and ends with the formation of the membrane attack complex that results in the disruption of the erythrocyte membrane, causing hemolysis. 

On the other hand, in the alternative pathway, C3 activation may occur spontaneously or via other alternative pathway activators. Nascent C3b may covalently binds to the erythrocyte membranes.  Binding of C3B proceeds via the alternative pathway until the formation of membrane attack complex and the destruction of erythrocyte membrane. 

Extravascular hemolysis constitutes the predominant form of AIHA.  It is caused by non-complement fixing antibodies or complement fixing antibodies but without going through the complement cascade.  It is usually associated with the presence of hyperbilirubinuria, elevation of lactate dehydrogenase, low serum haptoglobin - caused by immune adherence and phogocytosis mediated by reticuloendothelial cells of the liver or spleen.

Besides, AIHA can be categorized into idiopathic or secondary cases – two very broad categories.  Idiopathic AIHA is identified as the increased hemolysis that is apparently unaccompanied by any co-existing diseases.  On the other hand, secondary AIHA is always associated with a well defined, accompanying disease.

Looking at the clinical and laboratory features, AIHA can be divided into a few categories in which prognosis and management among themselves are vastly different. 

back to top

Warm-type antibody AIHA

About 70% of AIHA patients suffer from warm type AIHA of which approximately 50% has associated diseases (secondary AIHA).  Warm type AIHA involves IgG as the auto-antibodies, which, in this case is optimally active at 37°C.  Its pathogenic mechanism is poorly understood.

Patients of warm type AIHA suffer from fever, jaundice and hemoglobinuria.  Two types of IgGs are involved – complement fixing or non-complement fixing auto-IgGs.  Complement fixing auto-antibodies cause intravascular hemolysis and the immune clearance of erythrocytes via the liver.  On the other hand, non-complement fixing auto-antibodies result in extravascular hemolysis and the ensuing immune clearance via the spleen. 

The capability of auto-IgGs in inducing two different pathways of erythrocyte immune clearance was believed to be caused by the antigenic specificity of the IgG antibodies instead of the IgG subclass (Victoria et al, 1989).  Therefore, erythrocyte cell surface antigens are crucial in the determination of immune clearance.  Also, complement activation depends on the antigenic distribution of the various blood group antigens.   Antigens sparsely distributed on the erythrocyte surface are less likely to mediate complement activation as doublets of IgGs are needed for complement binding.

It was also shown that complement fixing auto-IgG is more efficient in mediating immune clearance than their non-complement fixing counterparts.  Besides, anti-Duffy blood group antibodies are able to elicit immune clearance better than anti-Rh antibodies.

The characteristic diagnostic test for warm type AIHA is the direct antiglobulin test (DAT) also known as Coombs’s Test.   In DAT, monospecific sera with only anti-IgG or anti-C3d antibodies are manufactured.  The monospecific anti-sera can cause agglutination when mixed with erythrocytes that carry appropriate globulin on their surface.  In warm type antibody AIHA patients, the red cells are always coated with Ig G or C3d.

Corticoid steroid such as prednisolone is used to treat initial AIHA.  More severe cases may require splenectomy, the removal of spleen.  Besides, taking immunosupressive drugs, plasma exchange and transfusion therapy are applied for more severe cases.

  back to top

Cold-type antibody AIHA

Cold type antibody AIHA is also known as cold agglutination syndrome (CAS) - characterized by the agglutination of blood, is most active in cold weather (< 4°C) and is caused by IgM autoantibodies.  It constitutes approximately 15% of all AIHA patients and is prevalent in old or middle-aged patients.  CAS patients suffer from pallor, jaundice, splenomegaly, chronic anemia and hemoglobinuria.  The degree of anemia incurred varies with the degree of cold exposure.  CAS patients will give a positive DAT test, especially in the cold (<4°C) due to the presence of C3d on their erythrocyte membrane.  However, IgM that is responsible for binding the complement is not shown in the Coombs’s test as antiglobulin serum does not readily detect IgM.

CAS is usually idiopathic but can be associated with Mycoplasma pneumoniae infection or infectious mononucleosis.  In contrast to warm type AIHA patients, CAS patients have a chronic course and can tolerate their mild or moderate anemia quite well.  CAS is not as severe with the warm counterpart and thus avoidance of cold exposure is the best way to avoid severe anemia.  

  back to top

Other AIHAs

Paradoxysmal AIHA is a rare disorder that constitutes only less than 1% of all AIHA cases.  It usually occurs after cold exposure in patients with congenital syphilis and is prevalent in children.  About 8% of AIHA patients suffer from combined cold-antibody and warm-antibody AIHA in which both cold type and warm type antibodies are present in their sera.  The cold type antibody is modestly elevated at 4°C but can react up to 30°C.  AIHA with negative DAT is another type of AIHA.  However, with a more sensitive assay like radiolabeled antiglobulin sera and agglutinating potentiators, the auto-antibodies can be detected.

   back to top

Biochemical Research

“Studies of the pathophysiology of autoimmune hemolytic anemia emphasize the important role of cell membrane receptors for various immunologically active proteins in the clearance of foreign or damaged particulate materials from the blood stream” (Frank et al, 1987). 

By using immunoprecipitation with autohemolytic autoantibodies with chymotrypsinized erythrocytes, Victoria et al discovered 2 fragments of proteins with similar molecular mass to the band-3 protein of erythrocyte membrane.  Further immunoblotting experiments with sheep anti-human band-3 protein antibody confirmed its identity.  Besides, with the use of I125 labeled auto-antibodies, they were able to show the consistency of binding behavior with respect to epitope localization of band-3.  The auto-antibodies bind to all types of erythrocytes being tested at a density of 106 per red cell. It was concluded that IgGs eluted from erythrocytes of AIHA patients consist predominantly of single totally erythrocyte absorbable antibodies and they bind to the band-3 protein of the erythrocyte membrane.  On the other hand, Wakui et al found a new autoantibody directing at band 4.1 of the erythrocyte membrane protein by using western blotting with erythrocyte membrane protein.  However, there were no protein 4.1 activities in six other AIHA patients, three other hemolytic anemia patients and other control subjects.  In addition to that, they were able to find two other auto-bodies that directs at the En-antigens and the S antigens.

Becton et al, reported an infant girl with severe AIHA that was caused by auto-antibodies directing against Vel antigen, a common antigen on the erythrocytes.  This disease is steroid resistant and blood transfusion with Vel-positive blood caused massive hemolysis.    Hochstrasser et al tried using the high resolution of two-dimensional gel electrophoresis to look at the plasma of a 63-year old AIHA patient and he successfully detected the overexpressed proteins in the plasma.  By using a heavier loading and cutting out the overexpressed protein spot for microsequencing, they were able to identify it as the κ (kappa) light chain of antibodies.

  back to top

Future Directions

AIHA is not yet sufficiently characterized.  There are more questions we may ask.  For instance, what is the receptor protein for AIHA antibodies?  What functions do these receptors play in erythrocytes?  What about the density distribution of the protein?  How does the binding of antibodies contribute to the hemolysis of erythrocytes?  What is the underlying etiology for auto-antibody production?  Why is it that the auto-antibody is only targeted to erythrocytes and destroy them instead of other types of cell?  We need well planned experiments before the mysteries of the AIHA can be unraveled.    

  back to top

First created on 25/3/2000. Last updated 5/4/2000