**3. Prevention of alloimmunization and improvement of transfusion therapy**

Prevention of alloimmunization is desirable for any blood transfusion; however, for patients not previously transfused or only having episodic blood transfusions, matching for all clinically significant antigens is not of great concern, but can result in alloimmunization against non-matched antigens. For patients previously transfused, particularly transfusion-dependent patients, the alloimmunization risk is higher and management of alloimmunized patients is of greater concern. Their alloimmunization status, including antigens of low clinical significance, is a critical part of their clinical history that may enable health care providers to take measures to prevent further alloimmunization.

As of May 2019, the International Society of Blood Transfusion (ISBT) recognizes 36 blood group systems. Of the 360 identified antigens, 322 have been assigned to a specific blood group system. These antigens have variable immunogenicity and not all blood group antigens are involved with the production of clinically significant antibodies after blood transfusion or pregnancy. Ideally, every blood transfusion should be compatible for the most clinically significant antigens in the RH (D, C, E, c, e), KEL (K), FY (Fya , Fyb ), JK (Jka , Jkb ), and the MNS (S, s) systems to prevent alloimmunization; however, the standard pre-transfusion cross-matching is only performed for ABO blood group and the Rh(D) antigen; ABO matching is performed to avoid acute hemolytic transfusion reactions caused by natural IgM antibodies against ABO antigens, and Rh(D) matching is performed because of the high immunogenicity of the Rh(D), which is implicated in DHTR and HDFN.

Currently, recommendations for partial and extended donor unit/patient matching are limited to specific groups including [1] patients on long-term transfusion protocol (i.e. SCD, MSD, thalassaemia, and aplastic anemia), [2] patients who have developed alloantibodies, and [3] patients with warm autoimmune hemolytic anemia.

*Human Blood Group Systems and Haemoglobinopathies*

the use of an incompatible unit in a given patient.

resources to find compatible RBC units [2].

**2. Blood transfusion and risk of RBC alloimmunization**

available for use.

product is not compromised; testing for infectious agents ensures a second layer of screening for pathogens that could be transmitted to the patient with an infected blood unit; testing for blood cell typing helps to avoid improper usage of the product since non-compatible blood can produce serious consequences and even fatalities; lastly, the transfusion process itself requires strict checkpoints to prevent

The scope of this chapter focuses on blood group typing and approaches to achieve better management and prevention of alloimmunization. We will provide an overview on the following topics: blood transfusion and risk of RBC alloimmunization; approaches for prevention of alloimmunization and improvement of transfusion therapy; serology and molecular typing methods and their use for RBC matching; current technologies for RBC genotyping; and standards for red cell antigens typing including scope, importance, and description of reference reagents

RBC transfusion is used for therapeutic treatment of various hematologic disorders including SCD, thalassaemia, MDS, autoimmune hemolytic anemia, and others. Management of patients may require lifelong regular transfusions for treatment of symptomatic anemia and prevention of disease complications. Although transfusion safety and donor-patient matching have improved over the years, RBC alloimmuni-

RBC alloimmunization is a serious adverse event of transfusions and can cause further clinical problems in the patients including worsening of anemia, development of autoantibodies, acute or delayed hemolytic transfusion reactions (DHTR), bystander hemolysis, organ failure, and cause serious complications during pregnancies. Frequent transfusions can lead to the production of multiple alloantibodies, which is often associated with autoantibodies requiring extensive serological workups and additional transfusions for proper treatment, increasing time and

Antibodies against ABO blood group antigens are naturally produced, IgM class, and are capable of rapid induction of intravascular destruction of RBCs by complement-mediated mechanism. Clinical outcome of a patient transfused with incompatible ABO blood can vary from no adverse effects to permanent organ damage and death depending on the volume of ABO-incompatible RBC transfused. As for non-ABO blood groups, clinically significant antibodies against non-ABO antigens are from IgG class and rarely activate complement. Instead, they cause DHTR or hemolytic disease of fetus and newborn (HDFN) by extravascular destruction of RBCs marked with IgG [3]. DHTR has been ranked as second or third most commonly reported cause of fatal transfusion reactions secondary to non-ABO antibodies in many countries including the United States [4–8]. However, experts believe that DHTR has been under-recognized or under-reported to biovigilance agencies and FDA, possibly because the reactions can be obscured by underlying disease (liver disease, massive trauma, and SCD), lack of knowledge among clinicians regarding the reactions, or lack of knowledge regarding the mechanism of

Reported RBC alloimmunization rates have considerable variations depending on the population and disease studied [9]. The rates are estimated between 1 and 3% in patients that receive episodic transfusions, while for patients who receive chronic blood transfusions like patients with SCD and MDS, rates vary between 8 and 76% [9–12]. Although the most commonly observed alloantibodies of clinical

zation, transmission of infections, and iron overload are still a concern [1].

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reporting [7].

Verification of compatibility for Rh (D, E, C, c, e) and K, which are the most frequent antigens involved in alloimmunization, is considered partial matching. Extended matching should include at least RH (D, C, E, c, e), KEL (K), FY (Fya , Fyb ), JK (Jk<sup>a</sup> , Jkb ), MNS (S, s) and, if available, additional antigens.
