ABO blood groups and galectins: Implications in transfusion medicine and innate immunity.

ABO blood group antigens, which are complex carbohydrate moieties, and the first human polymorphisms identified, are critical in transfusion medicine and transplantation. Despite their discovery over a century ago, significant questions remain about the development of anti-ABO antibodies and the structural features of ABO antigens that cause hemolytic transfusion reactions. Anti-ABO antibodies develop naturally during the first few months of life, in contrast to other red blood cell (RBC) alloantibodies which form after allogeneic RBC exposure. Anti-ABO antibodies are the most common immune barrier to transfusion and transplantation, but the factors driving their formation are incompletely understood. Some studies suggest that microbes that express glycans similar in structure to the blood group antigens could play a role in anti-blood group antibody formation. While the role of these microbes in clinically relevant anti-blood group antibody formation remains to be defined, the presence of these microbes raises questions about how blood group-positive individuals protect themselves against blood group molecular mimicry. Recent studies suggest that galectins can bind and kill microbes that mimic blood group antigens, suggesting a unique host defense mechanism against microbial molecular mimicry. However, new models are needed to fully define the impact of microbes, galectins, or other factors on the development of clinically relevant naturally occurring anti-blood group antibodies.

Galectin-4 Antimicrobial Activity Primarily Occurs Through its C-Terminal Domain.

Although immune tolerance evolved to reduce reactivity with self, it creates a gap in the adaptive immune response against microbes that decorate themselves in self-like antigens. This is particularly apparent with carbohydrate-based blood group antigens, wherein microbes can envelope themselves in blood group structures similar to human cells. In this study, we demonstrate that the innate immune lectin, galectin-4 (Gal-4), exhibits strain-specific binding and killing behavior towards microbes that display blood group-like antigens. Examination of binding preferences using a combination of microarrays populated with ABO(H) glycans and a variety of microbial strains, including those that express blood group-like antigens, demonstrated that Gal-4 binds mammalian and microbial antigens that have features of blood group and mammalian-like structures. Although Gal-4 was thought to exist as a monomer that achieves functional bivalency through its two linked carbohydrate recognition domains, our data demonstrate that Gal-4 forms dimers and that differences in the intrinsic ability of each domain to dimerize likely influences binding affinity. While each Gal-4 domain exhibited blood group-binding activity, the C-terminal domain (Gal-4C) exhibited dimeric properties, while the N-terminal domain (Gal-4N) failed to similarly display dimeric activity. Gal-4C not only exhibited the ability to dimerize but also possessed higher affinity toward ABO(H) blood group antigens and microbes expressing glycans with blood group-like features. Furthermore, when compared to Gal-4N, Gal-4C exhibited more potent antimicrobial activity. Even in the context of the full-length protein, where Gal-4N is functionally bivalent by virtue of Gal-4C dimerization, Gal-4C continued to display higher antimicrobial activity. These results demonstrate that Gal-4 exists as a dimer and exhibits its antimicrobial activity primarily through its C-terminal domain. In doing so, these data provide important insight into key features of Gal-4 responsible for its innate immune activity against molecular mimicry.

Prior immunization against an intracellular antigen enhances subsequent red blood cell alloimmunization in mice.

Antibodies against red blood cell (RBC) alloantigens can increase morbidity and mortality among transfusion recipients. However, alloimmunization rates can vary dramatically, as some patients never generate alloantibodies after transfusion, whereas others not only become alloimmunized but may also be prone to generating additional alloantibodies after subsequent transfusion. Previous studies suggested that CD4 T-cell responses that drive alloantibody formation recognize the same alloantigen engaged by B cells. However, because RBCs express numerous antigens, both internally and externally, it is possible that CD4 T-cell responses directed against intracellular antigens may facilitate subsequent alloimmunization against a surface RBC antigen. Here, we show that B cells can acquire intracellular antigens from RBCs. Using a mouse model of donor RBCs expressing 2 distinct alloantigens, we demonstrate that immune priming to an intracellular antigen, which would not be detected by any currently used RBC compatibility assays, can directly influence alloantibody formation after exposure to a subsequent distinct surface RBC alloantigen. These findings suggest a previously underappreciated mechanism whereby transfusion recipient responders may exhibit an increased rate of alloimmunization because of prior immune priming toward intracellular antigens.

Antibody-mediated antigen loss switches augmented immunity to antibody-mediated immunosuppression.

Antibodies against fetal red blood cell (RBC) antigens can cause hemolytic disease of the fetus and newborn (HDFN). Reductions in HDFN due to anti-RhD antibodies have been achieved through use of Rh immune globulin (RhIg), a polyclonal antibody preparation that causes antibody-mediated immunosuppression (AMIS), thereby preventing maternal immune responses against fetal RBCs. Despite the success of RhIg, it is only effective against 1 alloantigen. The lack of similar interventions that mitigate immune responses toward other RBC alloantigens reflects an incomplete understanding of AMIS mechanisms. AMIS has been previously attributed to rapid antibody-mediated RBC removal, resulting in B-cell ignorance of the RBC alloantigen. However, our data demonstrate that antibody-mediated RBC removal can enhance de novo alloimmunization. In contrast, inclusion of antibodies that possess the ability to rapidly remove the target antigen in the absence of detectable RBC clearance can convert an augmented antibody response to AMIS. These results suggest that the ability of antibodies to remove target antigens from the RBC surface can trigger AMIS in situations in which enhanced immunity may otherwise occur. In doing so, these results hold promise in identifying key antibody characteristics that can drive AMIS, thereby facilitating the design of AMIS approaches toward other RBC antigens to eliminate all forms of HDFN.

Blood group A enhances SARS-CoV-2 infection.

Among the risk factors for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), ABO(H) blood group antigens are among the most recognized predictors of infection. However, the mechanisms by which ABO(H) antigens influence susceptibility to COVID-19 remain incompletely understood. The receptor-binding domain (RBD) of SARS-CoV-2, which facilitates host cell engagement, bears significant similarity to galectins, an ancient family of carbohydrate-binding proteins. Because ABO(H) blood group antigens are carbohydrates, we compared the glycan-binding specificity of SARS-CoV-2 RBD with that of galectins. Similar to the binding profile of several galectins, the RBDs of SARS-CoV-2, including Delta and Omicron variants, exhibited specificity for blood group A. Not only did each RBD recognize blood group A in a glycan array format, but each SARS-CoV-2 virus also displayed a preferential ability to infect blood group A-expressing cells. Preincubation of blood group A cells with a blood group-binding galectin specifically inhibited the blood group A enhancement of SARS-CoV-2 infection, whereas similar incubation with a galectin that does not recognize blood group antigens failed to impact SARS-CoV-2 infection. These results demonstrated that SARS-CoV-2 can engage blood group A, providing a direct link between ABO(H) blood group expression and SARS-CoV-2 infection.

Marginal zone B cells mediate a CD4 T-cell-dependent extrafollicular antibody response following RBC transfusion in mice.

Red blood cell (RBC) transfusions can result in alloimmunization toward RBC alloantigens that can increase the probability of complications following subsequent transfusion. An improved understanding of the immune mechanisms that underlie RBC alloimmunization is critical if future strategies capable of preventing or even reducing this process are to be realized. Using the HOD (hen egg lysozyme [HEL] and ovalbumin [OVA] fused with the human RBC antigen Duffy) model system, we aimed to identify initiating immune factors that may govern early anti-HOD alloantibody formation. Our findings demonstrate that HOD RBCs continuously localize to the marginal sinus following transfusion, where they colocalize with marginal zone (MZ) B cells. Depletion of MZ B cells inhibited immunoglobulin M (IgM) and IgG anti-HOD antibody formation, whereas CD4 T-cell depletion only prevented IgG anti-HOD antibody development. HOD-specific CD4 T cells displayed similar proliferation and activation following transfusion of HOD RBCs into wild-type or MZ B-cell-deficient recipients, suggesting that IgG formation is not dependent on MZ B-cell-mediated CD4 T-cell activation. Moreover, depletion of follicular B cells failed to substantially impact the anti-HOD antibody response, and no increase in antigen-specific germinal center B cells was detected following HOD RBC transfusion, suggesting that antibody formation is not dependent on the splenic follicle. Despite this, anti-HOD antibodies persisted for several months following HOD RBC transfusion. Overall, these data suggest that MZ B cells can initiate and then contribute to RBC alloantibody formation, highlighting a unique immune pathway that can be engaged following RBC transfusion.

Storage differentially impacts alloimmunization to distinct red cell antigens following transfusion in mice.

INTRODUCTION: The impact of blood storage on red blood cell (RBC) alloimmunization remains controversial, with some studies suggesting enhancement of RBC-induced alloantibody production and others failing to observe any impact of storage on alloantibody formation. Since evaluation of storage on RBC alloimmunization in patients has examined antibody formation against a broad range of alloantigens, it remains possible that different clinical outcomes reflect a variable impact of storage on alloimmunization to specific antigens. METHODS: RBCs expressing two distinct model antigens, HEL-OVA-Duffy (HOD) and KEL, separately or together (HOD × KEL), were stored for 0, 8, or 14 days, followed by detection of antigen levels prior to transfusion. Transfused donor RBC survival was assessed within 24 h of transfusion, while IgM and IgG antibody production were assessed 5 and 14 days after transfusion. RESULTS: Stored HOD or KEL RBCs retained similar HEL or KEL antigen levels, respectively, as fresh RBCs, but did exhibit enhanced RBC clearance with increased storage age. Storage enhanced IgG antibody formation against HOD, while the oppositive outcome occurred following transfusion of stored KEL RBCs. The distinct impact of storage on HOD or KEL alloimmunization did not appear to reflect intrinsic differences between HOD or KEL RBCs, as transfusion of stored HOD × KEL RBCs resulted in increased IgG anti-HOD antibody development and reduced IgG anti-KEL antibody formation. CONCLUSIONS: These data demonstrate a dichotomous impact of storage on immunization to distinct RBC antigens, offering a possible explanation for inconsistent clinical experience and the need for additional studies on the relationship between RBC storage and alloimmunization.

Clodronate inhibits alloimmunization against distinct red blood cell alloantigens in mice.

BACKGROUND: Alloimmunization can be a significant barrier to red blood cell (RBC) transfusion. While alloantigen matching protocols hold promise in reducing alloantibody formation, transfusion-dependent patients can still experience RBC alloimmunization and associated complications even when matching protocols are employed. As a result, complementary strategies capable of actively preventing alloantibody formation following alloantigen exposure are warranted. STUDY DESIGN AND METHODS: We examined whether pharmacological removal of macrophages using clodronate may provide an additional strategy to actively inhibit RBC alloimmunization using two preclinical models of RBC alloimmunization. To accomplish this, mice were treated with clodronate, followed by transfusion of RBCs expressing the HOD (HEL, OVA, and Duffy) or KEL antigens. On days 5 and 14 post transfusion, anti-HOD or anti-KEL IgM and IgG antibodies were evaluated. RESULTS: Low dose clodronate effectively eliminated key marginal zone macrophage populations from the marginal sinus. Prior treatment with clodronate, but not empty liposomes, also significantly inhibited IgM and IgG anti-HOD alloantibody formation following transfusion of HOD RBCs. Similar exposure to clodronate inhibited IgM and IgG antibody formation following KEL RBC transfusion. CONCLUSIONS: Clodronate can inhibit anti-HOD and anti-KEL antibody formation following RBC transfusion in preclinical models. These results suggest that clodronate may provide an alternative approach to actively inhibit or prevent the development of alloantibodies following RBC transfusion, although future studies will certainly be needed to fully explore this possibility.

Erythropoietic properties of human induced pluripotent stem cells-derived red blood cells in immunodeficient mice.

Transfusion of red blood cells (RBCs) is a life-saving intervention for anemic patients. Human induced pluripotent stem cells (iPSC) have the capability to expand and differentiate into RBCs (iPSC-RBCs). Here we developed a murine model to investigate the in vivo properties of human iPSC-RBCs. iPSC lines were produced from human peripheral blood mononuclear cells by transient expression of plasmids containing OCT4, SOX2, MYC, KLF4, and BCL-XL genes. Human iPSC-RBCs were generated in culture supplemented with human platelet lysate, and were CD34- CD235a+ CD233+ CD49dlow CD71low ; about 13% of iPSC-RBCs were enucleated before transfusion. Systemic administration of clodronate liposomes (CL) and cobra venom factor (CVF) to NOD scid gamma (NSG) mice markedly promoted the circulatory survival of human iPSC-RBCs following transfusion. While iPSC-RBCs progressively decreased with time, 90% of circulating iPSC-RBCs were enucleated 1 day after transfusion (CD235a+ CD233+ CD49d- CD71- ). Surprisingly, human iPSC-RBCs reappeared in the peripheral circulation at 3 weeks after transfusion at levels more than 8-fold higher than at 1 h after transfusion. Moreover, a substantial portion of the transfused nucleated iPSC-RBCs preferentially homed to the bone marrow, and were detectable at 24 days after transfusion. These results suggest that nucleated human iPSC-derived cells that homed to the bone marrow of NSG mice retained the capability to complete differentiation into enucleated erythrocytes and egress the bone marrow into peripheral blood. The results offer a new model using human peripheral blood-derived iPSC and CL/CVF-treated NSG mice to investigate the development and circulation of human erythroid cells in vivo.

Seniors from United States allopathic medical schools matching into pathology residency, 2018-2022.

BACKGROUND: From 2008 to 2017, 28.8 % fewer United States allopathic medical students (MD seniors) applied to pathology residency in the Main Residency Match (MRM) and 27.5 % fewer matched. This study is a 5-year follow-up. METHODS: MRM data from 2018 to 2022 were reviewed to determine the numbers of MD seniors that applied and matched to pathology residency and other major medical specialties. RESULTS: From 2018 to 2022, the number of MD seniors applying to pathology increased 4.6 % from 237 to 248, while MD seniors matching to pathology increased 5.0 % from 220 to 231. For the 4 years from 2018 to 2021, there was a slight decline in MD seniors filling pathology positions, followed by a substantial 16.7 % spike in 2022. For the entire 5-year interval, because the number of filled pathology residency positions increased by 9.0 %, the percentage of filled positions taken by MD seniors declined from 38.7 % to 37.3 %. Of the 15 major medical specialties evaluated, pathology now has the 14th lowest percentage of filled positions taken by MD seniors. CONCLUSIONS: The number of MD seniors applying and matching to pathology residency increased over the past 5-years, in contrast to the timespan of 2008 to 2017. However, the percentage of pathology residency positions taken by MD seniors continued to decline and remains low compared to other major medical specialties. MRM data should be continually monitored to study trends in MD seniors filling pathology residency positions in the context of new recruitment efforts and the pandemic.

The medical student's guide to pathology residency, fellowships, and careers.

Recent studies have shown that relatively few MD, DO, and underrepresented in medicine (URM) students and physicians are matching into pathology residency in the United States (US). In the 2021 Main Residency Match, just 33.6% of filled pathology residency positions were taken by senior year students at US allopathic medical schools. This has been attributed to the fact that pathology is not a required rotation in most US medical schools, pathology is often taught in an integrated curriculum in the US where is does not stand out as a distinct field, and because the COVID-19 pandemic led to a suspension of in-person pathology rotations and electives. Ultimately, many US medical students fail to consider pathology as a career pathway. The objective of this article is to provide medical students with basic information, in the form of frequently asked questions (FAQs), about pathology training and career opportunities. This was accomplished by forming a team of MD and DO pathology attendings, pathology trainees, and a medical student from multiple institutions to create a pathology guide for medical students. This guide includes information about post-sophomore fellowships, 5 major pathology residency tracks, more than 20 fellowship pathways, and allopathic and osteopathic board examinations. This guide also contains photographs and descriptions of major pathology sub-specialties, including the daily and on-call duties and responsibilities of pathology residents. The exciting future of pathology is also discussed. This guide supports the agenda of the College of American Pathologists' (CAP) Pathologist Pipeline Initiative to improve student recruitment into pathology.

Pathology's historic 2019 incoming residents: Why "the internationalization of pathology" may markedly advance transfusion medicine and cellular therapeutics.

OBJECTIVES: This study had two objectives: (1) to determine if, in the United States of America (US), the proportion of non-US citizen international medical graduates (non-US IMGs) entering pathology residencies had increased (again) in 2019 and (2) to assess how this multi-year trend might impact transfusion medicine in the US. METHODS: The most recent (2019) "National Resident Matching Program" (NRMP) data were analyzed. To assess potential future impact, using controversies related to Plasmodium falciparum (Pf) malaria, conflicting US and non-US perspectives were reviewed. Differences between published US and non-US views were identified regarding, for example, the value of Pf-resistant ("variant") red blood cells (RBCs) and exchange transfusions. RESULTS: Year 2019 is the first year non-US IMGs were the largest group to fill residency-training positions for a major US specialty via the "Main Residency Match." Also notable, US and non-US views were found to differ markedly regarding (1) the value and safety of Pf-resistant RBC variants and exchange transfusions, and (2) the threat of drug-resistant Pf-malaria parasites. Non-US clinicians and researchers seem more concerned about Pf-malaria, and their interest in cellular therapies seems greater and more optimistic. CONCLUSIONS: In 2019, the historically high proportion of non-US IMGs among incoming pathology residents dramatically highlights the steady demographic shift that began years ago: "the internationalization of pathology" in the US. Fortunately, a review of publications related to exchange transfusion, Pf-malaria, and variant RBCs suggests non-US IMGs may markedly promote and advance cell therapies such as therapeutically-rational exchange (T-REX) of disease-resistant RBCs.

"Dual-gene" malaria-resistance: Therapeutically-rational exchange (T-REX) of group-O sickle trait and group-O C-traittrait red blood cells can be evaluated in Benin and Nigeria.

BACKGROUND: Using indicators of disease severity, clinicians can predict which Plasmodium falciparum (Pf) malaria patients being treated with artesunate or quinine are likely to die despite these drugs. Effective "rescue adjuncts" are needed when drugs alone are inadequate. "Therapeutically-rational exchange" (T-REX) of special malaria-resistant red blood cells (RBCs) has been proposed to optimize adjunctive exchange transfusion. METHODS: Studies were reviewed that (1) quantified how group-O status and "sickle-trait" (HbAS) and "C-trait" (HbAC) hemoglobins affect Pf mortality, risk of thrombosis, or birth outcomes for women with pregnancy associated malaria (PAM), (2) reported prevalences of "dual-gene" malaria-resistant RBCs, or (3) reflected the level of exchange-transfusion and malaria-related expertise in Benin and Nigeria. RESULTS: Data show that the malaria- and thrombosis-resistance of RBCs depend on specific genes and the patient's clinical status and medical history. In malaria-endemic Benin and Nigeria, prevalences of "dual-gene" malaria-resistant group-O HbAS and group-O HbAC RBCs are substantial, and both malaria- and exchange-related expertise are outstanding. CONCLUSIONS: T-REX of "dual-gene" malaria-resistant RBCs is feasible in Benin and Nigeria and warrants evaluation as a rescue adjunct for 3 subsets of Pf-malaria patients. For therapeutic use, group-O HbAS RBCs are likely to be more effective than non-O HbAS RBCs for Pf-infected patients who (1) have a history of thrombosis or (2) are taking birth-control hormones while group-O HbAC RBCs may substantially improve birth outcomes for women with PAM. Studies suggest it is prudent to assume - until proven otherwise - that T-REX of "dual-gene" malaria-resistant RBCs can improve ("personalize") rescue of these patient subsets.

Comparing leukapheresis protocols for an AML patient with symptomatic leukostasis.

BACKGROUND: Acute myeloid leukemia (AML) is a malignancy characterized by rapid clonal proliferation of myeloid precursors, which can result in hyperleukocytosis. Leukapheresis can be used to rapidly reduce the white blood cell count (WBC). However, the only FDA cleared device for WBC depletion, the COBE Spectra, will no longer be supported by the manufacturer in 2017, and there are few studies comparing different methods of leukapheresis. CASE REPORT: A 68-year-old African American female was admitted to the hospital for relapse of her AML. Laboratory data demonstrated a WBC count of 291 600/µL and flow cytometry of the peripheral blood demonstrated 85% myeloid blasts. Leukapheresis was ordered to help treat the leukostasis. METHODS: Three different apheresis protocols were used to achieve cytoreduction: Spectra Optia mononuclear collection (MNC) protocol, Spectra Optia granulocyte collection (PMN) protocol, and Therakos CELLEX buffy coat collection without return. Due to different inlet flow rates, the procedures were evaluated based on the number of WBCs collected and volume of blood processed (VBP). RESULTS: The Spectra Optia PMN collected the most WBCs and collected nearly as many WBCs per VBP as the Therakos CELLEX, which had the highest value. CONCLUSION: To our knowledge, we are reporting the first use of Therakos CELLEX and Spectra Optia PMN protocol for WBC depletion. While the Spectra Optia granulocyte protocol showed the best performance for this AML patient, further studies will be needed to compare the Spectra Optia PMN protocol to the MNC protocol for AML patients.

Validation of the PLASMIC score at a University Medical Center.

BACKGROUND: The PLASMIC score was recently described as a convenient tool for predicting ADAMTS13 activity ≤10% in patients with possible thrombotic thrombocytopenic purpura (TTP), while awaiting the results of this send-out test. The purpose of this study was to validate the PLASMIC score at our University Medical Center. METHODS: Apheresis records were reviewed from 2008 to 2017 to identify patients who received plasma exchange (PLEX) for suspected TTP. The ADAMTS13 activity and PLASMIC scoring criteria were recorded, and the PLASMIC score was calculated. RESULTS: Of the 41 patients identified, 20 met inclusion criteria, of which 7 patients had ADAMTS13 activity ≤10%. Intermediate and high PLASMIC scores had 100% sensitivity, 46.2% specificity, 50% positive predictive value (PPV), and 100% negative predictive value (NPV). CONCLUSION: These results are consistent with the original validation study of the PLASMIC score, supporting the efficacy of the PLASMIC score and validating its use at our institution.

Hematologic complications in a patient with Glycine soja polyagglutination following fresh frozen plasma transfusion.

CONCLUSIONS: Polyagglutination is a rare and underdiagnosed condition, characterized by agglutination of red blood cells(RBCs) with almost all ABO-compatible adult sera. Polyagglutination can occur when a cryptantigen is exposed on RBCs via microbial enzyme activity. Becausenearly all adults naturally produce antibodies against cryptantigens, transfusion of plasma can cause unexpected hemolysis and hematologic complications, such as thrombocytopenia and disseminated intravascular coagulation, in patients whose cryptantigens are exposed. We report a case of Glycine soja polyagglutination occurring in a 60-year-old African-American man with disseminated methicillin-resistant Staphylococcus aureus (MRSA) infection. Prior to transfusion, the patient developed severe anemia of unknown etiology. Following transfusion of 3 units of fresh frozen plasma (FFP), his RBC count could not be determined for 24 days because of RBC agglutination in his blood sample. In addition, the FFP transfusion correlated with the rapid development of severe, transfusionrefractory thrombocytopenia and anemia. The perplexed clinical team consulted the blood bank. A direct antiglobulin test demonstrated 1+ mixed-field reactivity with both monoclonal anti-IgG and anti-C3d. Lectin panel testing showed reactivity with only Glycine soja, confirming the condition. Subsequently, plasma components were avoided, and RBC and platelet (PLT) components were washed prior to transfusion. After a 44-day hospitalization involving the transfusion of 22 units of RBCs and 13 units of PLTs, the patient was discharged to a long-term care facility. The patient's confounding hematologic complications can best be explained by polyagglutination, which developed secondary to the severe MRSA infection. The FFP transfusion likely passively transferred antibodies that bound to the patient's RBC cryptantigens, leading to RBC agglutination and anemia. The development of severe thrombocytopenia may be related to cryptantigen exposure on the patient's PLTs. Although difficult to identify, polyagglutination needs to be recognized to appropriately manage hemotherapy. The purpose of this case study is to report hematologic complications following FFP transfusion in a patient with Glycine soja polyagglutination, a rarely described condition.

A suspected delayed hemolytic transfusion reaction mediated by anti-Joa.

CONCLUSIONS: A 32-year-old African-American woman with a history of sickle cell disease presented for surgical evaluation of left total hip arthroplasty due to avascular necrosis of the femoral head. In anticipation of a complex orthopedic procedure, pre-surgical blood work was ordered. The patient's Fenwal blood sample typed as group O, D+. Although the patient had a history of anti-Fya, the antibody identification was inconclusive, so the workup was sent to a reference laboratory. The patient was last transfused with red blood cells (RBCs) 2 years earlier, but had no history of transfusion reactions. Due to surgery, the patient's hemoglobin (Hb) decreased from 10.2 g/dL (preoperative) to 8.6 g/dL (postoperative). One unit of weakly crossmatch-incompatible Fy(a-), C-, E-, K-, and sickle cell hemoglobin S (HbS)-negative RBCs was transfused without incident, and the patient was discharged. Several days later, the reference lab reported two new specificities, anti-Joa and anti-Jkb. Fortunately, the transfused RBC unit was Jk(b-). Therefore, the crossmatch incompatibility was attributed to anti-Joa, which targets a high-prevalence antigen found in 100 percent of most populations. Two weeks after discharge, the patient returned in sickle vaso-occlusive pain crisis. The patient was clinically stable, but her Hb was 6.7 g/dL. One unit of Fy(a-), Jk(b-), C-, E-, K-, HbS- RBCs, which was weakly crossmatch-incompatible, was transfused. The following day, her Hb was unchanged, lactic acid dehydrogenase increased from 951 to 2464 U/L, potassium increased from 3.7 to 4.6 mEq/L, creatinine increased from 0.60 to 0.98 mg/dL, and the patient developed a 38.4°C fever. These findings are consistent with a delayed hemolytic transfusion reaction (DHTR), mediated by anti-Joa, occurring 2 weeks after the first RBC transfusion. Further care could not be provided because the patient left the hospital against medical advice. The purpose of this case study is to report findings consistent with a DHTR attributed to anti-Joa, an antibody with relatively unknown clinical significance.