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Topic_1_AAAARoseff_2009_SickleCellDisease_review

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25 C e l e b r a t i n g Y e a r s Journal of Blood Group Serology and Education Volume 25, Number 2, 2009 Immunohematology
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IMMUNOHEMATOLOGY, Volume 25, Number 2, 2009 67 Sickle cell disease: a review S.D. Roseff* S ickle cell disease (SCD) is described as the first identi - fied “molecular” disease since its manifestations stem from a substitution of valine for glutamic acid in the structure of the β chain hemoglobin molecule. 1 As a result of this change, RBCs form characteristic “sickle” shapes and the surface of these RBCs attract each other, polymerizing when in a low oxygen environment. This seemingly “small” variation in the structure of the RBC causing polymerization leads to manifestations such as chronic occlusion of blood vessels (vaso-occlusion), reduced blood flow to vital organs (ischemia), and alterations of the immune system. In ad- dition, the abnormal sickle cells are prematurely removed from circulation, resulting in hemolytic anemia. Transfu- sion is a vital component of the treatment of some of the complications of SCD. It is also a modality used to prevent some of these complications from occurring. Patients with SCD are unique because those who are transfused usually require chronic transfusion, resulting in exposure to many different blood donors over the course of treatment. In addition, most patients with SCD in the United States are African American, and most donors are Caucasian from Western European descent. As a result of this difference, patients with SCD are exposed to RBC antigens that they lack, putting them at risk for forming alloantibodies, de- fined as RBC alloimmunization. 2 Therefore, it is important to understand the issues involved in the safe and effective transfusion of patients with SCD. Defining Hemoglobinopathies Hemoglobin (Hgb) is made up of iron (heme) and 4 globin chains. The type of globin chains determines the type of hemoglobin (see Table 1). 3 Since SCD is characterized by the presence of an abnor- mal or variant hemoglobin, hemoglobin S, it is characterized as a hemoglobinopathy. The composition of hemoglobin for patients with homozygous SS is presented in Table 2. There are a variety of other abnormal hemoglobins that may be present with or without Hgb S. The type of hemo- globin a person has is based on patterns of inheritance. If each parent contributes hemoglobin S, the child can inherit two copies and is designated as Hgb SS, or is homozygous for hemoglobin S. If a child only inherits one copy from one parent and a copy of normal hemoglobin, Hgb A, they are designated as Hgb AS, or heterozygous. These individuals are usually asymptomatic, and only develop manifestations under rare circumstances, where they become hypoxic, such as at high altitudes. Therefore, not every person with an abnormal hemoglobin develops or exhibits signs and symptoms. Although Hgb S is found worldwide, it is most commonly found in western Africa. About one in every 400– 500 African Americans, or 80,000, has SCD. About 9000 African Americans, or one in 12, have sickle cell trait.
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