Chapter 24: Hemolytic Anemia Resulting from Warm-Reacting Antibodies

• In autoimmune hemolytic anemia (AHA), shortened red blood cell (RBC) survival is the result of host antibodies that react with autologous RBC.

• AHA may be classified by whether an underlying disease is present (secondary) or not (primary or idiopathic) (Table 24–1).

• AHA may also be classified by the nature of the antibody (Table 24–2).

• "Warm-reacting" antibodies are usually of the IgG type, have optimal activity at 37°C, and bind complement.

• "Cold-reacting" antibodies show affinity at lower temperatures (see Chap. 25).

• Occasionally, mixed disorders occur with both warm and cold antibodies.

• Warm antibody AHA is the most common type.

• AHA occurs in all age groups, but the incidence rises with age, probably because the frequency of lymphoproliferative malignancies increases with age.

• In primary AHA, the autoantibody often is specific for a single RBC membrane protein, suggesting that an aberrant immune response has occurred to an autoantigen or a similar immunogen; a generalized defect in immune regulation is not seen.

• In secondary AHA, the autoantibody most likely develops from an immunoregulatory defect.

• Certain drugs (e.g., α-methyldopa) can induce specific antibodies in otherwise normal individuals by some unknown mechanism. These subside spontaneously when the drug is stopped.

• The red cells of some apparently normal individuals may be found coated with warm-reacting autoantibodies similar to those of patients with AHA. Such antibodies are noted in otherwise normal blood donors at a frequency of 1 in 10,000. A few develop AHA.

• RBC autoantibodies in AHA are pathogenic.

• RBCs that lack the targeted antigen have a normal survival.

• Transplacental passage of autoantibodies to a fetus can cause hemolytic anemia.

• Antibody-coated RBCs are trapped by macrophages primarily in the spleen, where they are ingested and destroyed or partially phagocytosed and a spherocyte with a lower surface area:volume ratio is released.

• Macrophages have cell surface receptors for the Fc portion of IgG and for fragments of C3 and C4b. These immunoglobulin and complement proteins on the RBC surface can act cooperatively as opsonins and enhance trapping of RBC.

• Large quantities of IgG, or the addition of C3b, will increase trapping of RBC by macrophages in the liver and spleen.

• Direct RBC lysis by complement is unusual in warm antibody AHA, probably as a result of interference with complement activity by several mechanisms. Lysis by complement is seen in cold antibody type AHA and paroxysmal cold hemoglobinuria (see Chap. 25).

• RBC may be destroyed by monocytes or lymphocytes by direct cytotoxic activity, without phagocytosis. The proportion of hemolysis caused by this mechanism is unknown. Antibodies may also attach to late erythroid precursors and suppress erythropoiesis.

• Generally, symptoms of anemia draw attention to the disease, although jaundice may also be a presenting complaint.

• Symptoms are usually slow in onset, but rapidly developing anemia can occur.

• Uncommonly severe anemia may require urgent care. The patient may display air-hunger, profound pallor, and weakness. This syndrome can be seen in patients with AHA in the setting of chronic lymphocytic leukemia.

• Physical examination may be normal if the anemia is mild. Splenomegaly is common but not always observed. Jaundice and physical findings related to more pronounced anemia may be noticed.

• AHA may be aggravated or first noticed during pregnancy. Both mother and fetus generally fare well if the condition is treated early.

General

• Anemia can range from mild to life-threatening.

• Blood film reveals polychromasia (indicating reticulocytosis) and spherocytes (see Fig. 24–1).

• With severe cases, nucleated RBC, RBC fragments, and, occasionally, erythrophagocytosis by monocytes may be seen (see Fig. 24–1).

• Reticulocytosis is usually present if the marrow has not been injured by some other condition; initially, a short period of relative reticulocytopenia occurs in one-third of the cases.

• Most patients have mild neutrophilia and normal platelet count, but occasionally immune neutropenia and thrombocytopenia occur concomitantly.

• Evans syndrome is a rare condition in which both autoimmune-mediated RBC and platelet destruction occur. A low neutrophil count may also be present.

• Marrow examination usually reveals erythroid hyperplasia; occasionally an underlying lymphoproliferative disease may be uncovered.

• Unconjugated hyperbilirubinemia is often present, but usually the total bilirubin level does not exceed 5 mg/dL, with less than 15 percent conjugated.

• Haptoglobin levels are usually low, and serum lactic acid dehydrogenase (LDH) activity is elevated.

• Urinary urobilinogen is routinely increased, but hemoglobinuria is uncommon.

Serologic Features

• The diagnosis of AHA requires demonstration of immunoglobulin and/or complement bound to the RBC (Table 24–2).

• This is achieved by the direct antiglobulin test (DAT) in which rabbit antiserum to human IgG or complement is added to suspensions of washed patient's RBC. Agglutination of the RBC signifies the presence of surface IgG or complement.

• The DAT is first performed with broad-spectrum reagents, including antibodies against both complement and immunoglobulin. If this is positive, further testing is done to define the offending antibody or complement component.

• RBC may be coated with:

  — IgG alone.

  — IgG and complement.

  — Complement only.

• Rarely, anti-IgA and anti-IgM reactions are encountered.

• Autoantibody exists in a dynamic equilibrium between RBC and plasma.

• Free autoantibody may be detected by the indirect antiglobulin test (IAT) in which the patient's serum is incubated with normal donor RBC, which are then tested for agglutination by the addition of antiglobulin serum.

• Binding affinity for antibodies varies, but in general, serum autoantibody is detectable in those with heavily coated RBC.

• A positive indirect test with a negative direct test probably does not indicate autoimmune disease but an alloantibody generated by a prior transfusion or pregnancy.

• Occasional patients exhibit all the features of AHA but have a negative DAT. The amount of their RBC-bound autoantibody is too low for detection by DAT but can often be demonstrated by more sensitive methods, such as enzyme-linked immunoassay or radioimmunoassay.

• The relationship between the amount of bound antibody and degree of hemolysis is variable.

• Subclasses IgG₁ and IgG₃ are generally more effective in causing hemolysis than IgG₂ and IgG₄, apparently because of greater affinity of macrophage Fc receptors for these subclasses as well as increased complement fixation abilities.

• Autoantibodies from AHA patients usually bind to all the types of RBC used for laboratory screening and therefore appear to be "nonspecific."

• However, the autoantibodies from individual patients usually react with antigens that are present on nearly all RBC types, the so-called "public" antigens, and only appear to lack specificity.

• Nearly half of the antibodies have specificity for epitopes on Rh proteins (Rh related) and hence will not react with cells of the rare Rh-null type.

• The remaining autoantibodies have a variety of specificities, but many are not defined.

• Other conditions may have spherocytosis, including hereditary spherocytosis, Zieve syndrome, Wilson disease, and clostridial sepsis. DAT is negative in these conditions.

• AHA and autoimmune thrombocytopenia may also occur as a manifestation of systemic lupus erythematosus (secondary AHA).

• Paroxysmal nocturnal hemoglobinuria and microangiopathic hemolytic anemia should also be considered, but minimal or no spherocytosis is seen and the DAT is negative.

• If the DAT is positive for complement alone, further serologic characterizations are warranted to distinguish cold-reacting from warm-reacting autoantibodies.

• In recently transfused patients, alloantibody against donor RBC may be detected by a positive DAT.

• Organ transplant recipients may develop a picture of AHA usually when an organ from a blood group O donor is transplanted into a group A recipient, probably because B lymphocytes persist in the transplanted organ and form alloantibodies against host RBC.

• Marrow transplant patients of blood group O who receive blood group A or B marrow may develop a briefly positive DAT, and RBC synthesized by the engrafted marrow may be hemolyzed until previously made recipient anti-A or anti-B disappears.

• Mixed chimera also occurs so that the immunocompetent host B lymphocyte continues to generate alloantibodies.

• Occasional patients have a positive DAT but minimal hemolysis and stable hematocrit. These patients need no treatment but should be observed for possible progression of the disease.

Transfusion

• Generally, anemia develops slowly so that RBC transfusion is not required; however, for rapid hemolysis or patients otherwise compromised (i.e., cardiac disease), transfusion may be lifesaving.

• Virtually all units are incompatible on cross-match unless one has an autoantibody that is specific for a single RBC antigen and RBC units lacking that antigen can be obtained.

• Transfused RBCs are destroyed as fast as or faster than host RBC but may tide the patient through a dangerous time.

• The blood bank should try to ascertain the ABO type of patient's RBC to avoid alloantibody-mediated hemolysis of donor cells.

Glucocorticoids

• Glucocorticoids slow or stop hemolysis in two-thirds of the patients.

• Twenty percent of patients will achieve a complete remission.

• Ten percent will show little or no response.

• Best results are seen in patients with primary AHA or AHA secondary to lupus erythematosus.

• Initial treatment should be with oral prednisone at 60 to 100 mg/d, orally.

• For the gravely ill, intravenous methylprednisolone at 100 to 200 mg in divided doses over the first 24 hours can be given.

• When the hematocrit stabilizes, prednisone may be slowly tapered to 15 to 20 mg/d at a rate of about 5 mg per week and continued for 2 to 3 months before slowly tapering off the drug entirely. In some cases in which tapering cannot be completed, alternative day therapy may be tried, 20 to 30 mg every other day by mouth.

• Relapses are common, and the patient should be closely monitored.

• The mechanism(s) of action of glucocorticoids in AHA has not been fully established but presumably they impair macrophage ingestion of antibody-coated RBC, early after treatment is started and suppress autoantibody production.

Splenectomy

• In patients who cannot be tapered off prednisone (approximately one-third), splenectomy is the next modality of therapy to use. If response is slow and the anemia is severe, splenectomy should be considered.

• Splenectomy removes the main site of RBC destruction. Hemolysis can continue, but much higher levels of RBC-bound antibody are necessary to cause the same rate of destruction. Sometimes the amount of cell-bound antibody will decrease after splenectomy, but often no change is noted.

• Approximately two-thirds of patients have complete or partial remission after splenectomy, but relapses frequently occur. If glucocorticoids are still necessary, it is often possible to use a lower dosage.

• Splenectomy slightly increases the risk of pneumococcal sepsis (children more than adults), and pneumococcal vaccine should be given several weeks before surgery, if feasible. In addition, prophylactic oral penicillin is often given to children after splenectomy.

Rituximab

• A monoclonal antibody directed against CD20 may be used to treat AHA based on its ability to eliminate B lymphocytes producing autoantibodies to RBCs. The rapid response in many patients in whom autoantibody is still circulating makes that an unlikely initial mechanism.

• Opsonized B lymphocytes may decoy macrophages and monocytes from autoantibody complexes and normalize autoreactive T lymphocyte responses.

• The response rate has averaged about 60 percent of patients treated with anti-CD20.

Immunosuppressive Drugs

• Either cyclophosphamide (60 mg/m²) or azathioprine (80 mg/m²) given daily can be used. Close attention to blood counts is crucial because erythropoiesis can be suppressed, temporarily worsening the anemia. Treatment can be continued for up to 6 months awaiting a response, and then tapered if and when the desired response is attained.

Other Treatments

• Plasmapheresis has been used with occasional success reported, but its efficacy is unpredictable.

• Variable success has been achieved with high-dose intravenous immunoglobulin (400 mg/kg daily for 5 days), danazol, 2-chlorodeoxyadenosine, thymectomy in children, and administration of vinblastine-loaded RBC.

• Idiopathic warm-antibody AHA runs an unpredictable course characterized by remissions and relapses.

• Survival at 10 years is approximately 70 percent.

• In addition to anemia, deep venous thrombosis, pulmonary emboli, splenic infarcts, and other cardiovascular events occur during active disease.

• In secondary warm-antibody AHA, prognosis is related to the underlying disease.

• Overall mortality rate in children is lower than in adults, ranging from 10 to 30 percent.

• AHA related to infection is self-limited and responds well to glucocorticoids.

• Children who develop chronic AHA tend to be older.