Immunohistochemistry assessment of tissue neutrophil-to-lymphocyte ratio predicts outcomes in melanoma patients treated with anti-programmed cell death 1 therapy.

Elevated neutrophil-to-lymphocyte ratio (NLR) is associated with diminished immunotherapy response in metastatic melanoma. Although NLR assessment in peripheral blood is established, tissue dynamics remain insufficiently explored. This study aimed to evaluate tissue NLR (tNLR)'s predictive potential through immunohistochemistry in immunotherapy-treated melanoma. Fifty melanoma patients who underwent anti-programmed cell death 1 (PD-1) therapy were assessed. Hematological, clinical and tumor features were collected from medical records. Responses were categorized using the Response Evaluation Criteria in Solid Tumors for immunotherapy (iRECIST) guidelines. Immunohistochemistry for tumor-infiltrating T cells (cluster differentiation 3) and neutrophils (myeloperoxidase) was performed on formalin-fixed paraffin-embedded tumor samples. NLR, derived NLR (dNLR) and tNLR were calculated. Overall survival (OS) and survival following immunotherapy (SFI) were calculated from diagnosis or immunotherapy start to loss of follow-up or death. Patients with high tNLR presented improved OS ( P =  0.038) and SFI with anti-PD-1 therapy ( P =  0.006). Both NLR and dNLR were associated with OS ( P =  0.038 and P =  0.046, respectively) and SFI ( P =  0.001 and P =  0.019, respectively). NLR was also associated with immunotherapy response ( P =  0.007). In conclusion, tNLR emerged as a novel potential biomarker of enhanced survival post anti-PD-1 therapy, in contrast to classical NLR and dNLR markers.

Real-time assessment of neutrophil metabolism and oxidative burst using extracellular flux analysis.

Neutrophil responses are critical during inflammatory and infective events, and neutrophil dysregulation has been associated with poor patient outcomes. Immunometabolism is a rapidly growing field that has provided insights into cellular functions in health and disease. Neutrophils are highly glycolytic when activated, with inhibition of glycolysis associated with functional deficits. There is currently very limited data available assessing metabolism in neutrophils. Extracellular flux (XF) analysis assesses real time oxygen consumption and the rate of proton efflux in cells. This technology allows for the automated addition of inhibitors and stimulants to visualise the effect on metabolism. We describe optimised protocols for an XFe96 XF Analyser to (i) probe glycolysis in neutrophils under basal and stimulated conditions, (ii) probe phorbol 12-myristate 13-acetate induced oxidative burst, and (iii) highlight challenges of using XF technology to examine mitochondrial function in neutrophils. We provide an overview of how to analyze XF data and identify pitfalls of probing neutrophil metabolism with XF analysis. In summary we describe robust methods for assessing glycolysis and oxidative burst in human neutrophils and discuss the challenges around using this technique to assess mitochondrial respiration. XF technology is a powerful platform with a user-friendly interface and data analysis templates, however we suggest caution when assessing neutrophil mitochondrial respiration.

Assessment of neutrophil to lymphocyte ratio, C-reactive protein, mean platelet volume in obese, and nonobese patients with polycystic ovary syndrome.

Polycystic ovary syndrome (PCOS) is associated with low-grade chronic inflammation. This was a retrospective case-control study. In the present study, the risk coefficients of neutrophil to lymphocyte ratio (NLR), high-sensitive C-reactive protein (hs-CRP), and mean platelet volume (MPV) in obese patients with PCOS were determined. This study was designed to investigate NLR, hs-CRP, and MPV levels in 68 obese patients with PCOS and 44 nonobese patients with PCOS, and our study group was matched with 47 obese and 43 nonobese controls, respectively. PCOS group had higher MPV, NLR, insulin, glucose, and HOMA-IR rates than those of the controls. Subgroup analyses revealed that the obese PCOS group had higher NLR, hs-CRP, and MPV levels compared to those of controls. The obese PCOS group had higher NLR, hs-CRP, and MPV levels compared to those of the nonobese PCOS group. The odds ratios and 95% confidence intervals of those variables (NLR, hs-CRP, MPV) were found significant (P < .05). NLR, hs-CRP, and MPV variables were found statistically significant in the analysis of receiver operating characteristics. Our study demonstrated that NLR, hs-CRP, and MPV levels are increased in patients with obese PCOS.

Preclinical safety assessment of a therapeutic human papillomavirus DNA vaccine combined with intravaginal interleukin-7 fused with hybrid Fc in female rats.

This study was conducted to establish the toxicological profile of combination treatment with therapeutic HPV DNA vaccines (GX-188E) and the long-acting form of recombinant human interleukin-7 fused with hybrid Fc (IL-7hyFc). GX-188E was administered intramuscularly by electroporation with or without IL-7hyFc intravaginally once per 2 weeks for 8 weeks (five times) in female Sprague-Dawley rats. Because up-regulation of immune responses and migration of antigen-specific T cells in cervicoviginal tissue were predicted as therapeutic effects, we distinguished adverse effects from therapeutic effects based on the severity of the systemic immune response, reversibility of lymphoid tissue changes, target tissue damage, and off-target immune responses. We observed that the number of neutrophils was increased, and the number of lymphocytes was decreased in the blood. Further, myofiber degeneration, necrosis, fibroplasia, and cell infiltration were observed at the GX-188E administration site. These changes were fully or partially recovered over a 4-week period. Analysis of lymphocytes in spleen revealed that CD4+ T cells and total T cells decreased in rats treated with GX-188E in combination with a high dose of IL-7hyFc (1.25 mg/animal). However, these changes were not considered adverse because they were transient and may have been related to electroporation-mediated DNA delivery or the local migration of lymphocytes induced by IL-7. Therefore, the potential toxicity of the combination of GX-188E and IL-7hyFc treatment was comparable to that of GX-188E treatment alone, and the no observed adverse effect level for GX-188E with IL-7hyFc was considered as 320 µg/animal for GX-188E and 1.25 mg/animal for IL-7hyFc.

Similarities and differences between HIV and SARS-CoV-2.

In the last 50 years we have experienced two big pandemics, the HIV pandemic and the pandemic caused by SARS-CoV-2. Both pandemics are caused by RNA viruses and have reached us from animals. These two viruses are different in the transmission mode and in the symptoms they generate. However, they have important similarities: the fear in the population, increase in proinflammatory cytokines that generate intestinal microbiota modifications or NETosis production by polymorphonuclear neutrophils, among others. They have been implicated in the clinical, prognostic and therapeutic attitudes.

Assessment of Interleukin-8 in Bronchial Asthma in Iraq.

It has been approved that neutrophils are responsible for many inflammatory lung diseases, such as acute respiratory distress syndrome, chronic obstructive pulmonary disease, and asthma. It is well documented that the CXC chemokine interleukin-8 (IL-8) plays a key role as a potent neutrophil recruiting and activating factor. Asthma is one of the most common major non-contagious diseases and has a substantial impact on the patient's quality of life. The current evidence suggests that asthma is a complex multifactorial disorder, and its etiology is increasingly attributed to interactions between genetic susceptibility, host factors, and environmental exposures. IL-8 plays an important role in respiratory diseases and is a known regulator of pulmonary inflammation and immunity, induced phagocytosis, and promoted angiogenesis. This study aimed to investigate the IL-8 gene expression in blood samples of bronchial asthma patients. Therefore, the blood samples were taken from two groups of participants, including the group of patients with asthma (n=100) in the age range of20-61years and the group of healthy individuals (n=50).The obtained results indicated that the expression of IL-8 mRNA in the group of asthma patients was three times higher than that in the group of healthy individuals. Therefore, it is suggested that the antagonism of IL-8 could be a potent therapeutic strategy in the treatment of asthma.

A Proteomics-Based Assessment of Inflammation Signatures in Endotoxemia.

We have previously shown that multimers of plasma pentraxin-3 (PTX3) were predictive of survival in patients with sepsis. To characterize the release kinetics and cellular source of plasma protein changes in sepsis, serial samples were obtained from healthy volunteers (n = 10; three time points) injected with low-dose endotoxin (lipopolysaccharide [LPS]) and analyzed using data-independent acquisition MS. The human plasma proteome response was compared with an LPS-induced endotoxemia model in mice. Proteomic analysis of human plasma revealed a rapid neutrophil degranulation signature, followed by a rise in acute phase proteins. Changes in circulating PTX3 correlated with increases in neutrophil-derived proteins following LPS injection. Time course analysis of the plasma proteome in mice showed a time-dependent increase in multimeric PTX3, alongside increases in neutrophil-derived myeloperoxidase (MPO) upon LPS treatment. The mechanisms of oxidation-induced multimerization of PTX3 were explored in two genetic mouse models: MPO global knock-out (KO) mice and LysM Cre Nox2 KO mice, in which NADPH oxidase 2 (Nox2) is only deficient in myeloid cells. Nox2 is the enzyme responsible for the oxidative burst in neutrophils. Increases in plasma multimeric PTX3 were not significantly different between wildtype and MPO or LysM Cre Nox2 KO mice. Thus, PTX3 may already be stored and released in a multimeric form. Through in vivo neutrophil depletion and multiplexed vascular proteomics, PTX3 multimer deposition within the aorta was confirmed to be neutrophil dependent. Proteomic analysis of aortas from LPS-injected mice returned PTX3 as the most upregulated protein, where multimeric PTX3 was deposited as early as 2 h post-LPS along with other neutrophil-derived proteins. In conclusion, the rise in multimeric PTX3 upon LPS injection correlates with neutrophil-related protein changes in plasma and aortas. MPO and myeloid Nox2 are not required for the multimerization of PTX3; instead, neutrophil extravasation is responsible for the LPS-induced deposition of multimeric PTX3 in the aorta.

Understanding Oxidation Propensity in GCSF and Assessment of its Safety and Efficacy.

PURPOSE: To understand the impact of methionine oxidation in GCSF on efficacy (neutrophil production/activation) and safety (biochemical and histopathological changes). METHODS: Nine GCSF biosimilars were analyzed for the levels of residual iron and copper content. Oxidation in GCSF was induced by H2O2 treatment and four samples were prepared: wtGCSF (no oxidation), MetO (1138), MetO (1,138,127) and MetO (1138,127,122). These samples were used to evaluate binding affinity with the GCSF receptor (GCSFR) using biolayer interferometry, thermal stability using circular dichroism and in vitro potency using a relevant cell-based assay. In vivo pharmacodynamics examined changes in neutrophil production upon GCSF methionine oxidation, with the outcome correlated with the differential expression of genes implicated in the GCSF mediated neutrophil activation/ maturation. Pre-clinical safety studies including biochemical and histopathological changes were also performed. RESULTS: Met 122 and Met 127 have the most deleterious effect on the potency. Lower binding affinity with GCSFR was identified as the underlying cause for lower efficacy and potency. Role of Asp 110 in GCSF as the critical residue having adverse impact on efficacy in context of methionine oxidation has been elucidated. Impairment of in vitro binding affinity with GCSF manifests as in vivo pharmacodynamic differences via differential expression of downstream genes required for neutrophil maturation. CONCLUSION: The data from the present study suggests that methionine oxidation in GCSF is a critical quality attribute that needs careful monitoring and control during commercial manufacturing and subsequent supply chain stages.

Assessment of Arf6 Deletion in PLB-985 Differentiated in Neutrophil-Like Cells and in Mouse Neutrophils: Impact on Adhesion and Migration.

Chemoattractant sensing, adhesiveness, and migration are critical events underlying the recruitment of neutrophils (PMNs) to sites of inflammation or infection. Defects in leukocyte adhesion or migration result in immunodeficiency disorders characterized by recurrent infections. In this study, we evaluated the role of Arf6 on PMN adhesion in vitro and on migration to inflammatory sites using PMN-Arf6 conditional knockout (cKO) mice. In PMN-like PLB-985 silenced for Arf6 fMLP-mediated adhesion to the ß2 integrin ligands, ICAM-1 and fibrinogen or the ß1/ß2 integrin ligand fibronectin was significantly reduced. Furthermore, overexpression of wild-type Arf6 promoted basal and fMLP-induced adhesion to immobilized integrin ligands, while overexpression of the dominant-negative Arf6 has the opposite effects. Using the Elane-Cre deleting mouse strains, we report that the level of Arf6 deletion in inflammatory PMNs isolated from the dorsal air pouches was stronger when compared to naïve cells isolated from the bone marrow. In PMN-Arf6 cKO mice, the recruitment of PMNs into the dorsal air pouch injected with LPS or the chemoattractant fMLP was significantly diminished. Impaired cell migration correlated with reduced cell surface expression of CD11a and CD11b in Arf6 cKO PMNs. Our results highlight that Arf6 regulates the activity and possibly the recycling of PMN integrins, and this compromises PMN migration to inflammatory sites.

Assessment of Neutrophil Apoptosis.

The process of neutrophil apoptosis has an important role in the resolution of acute inflammation. Apoptotic cell death is characterized by a coordinated sequence of cellular alterations that serve to uncouple neutrophil effector functions whilst maintaining plasma membrane integrity. In this way the release on neutrophil intracellular contents, including proteases, glycosidases, and reactive oxygen species, is limited during apoptosis. In addition, plasma membrane alterations associated with neutrophil apoptosis provide molecular cues that enable recognition by phagocytic cells, including macrophages. The recognition and uptake of apoptotic neutrophils by macrophages dampens proinflammatory responses to pathogen- or damage-associated molecular patterns and triggers release of proresolution mediators, that further promote resolution of inflammation. The key cellular and molecular events that act to control neutrophil apoptosis and subsequent macrophage phagocytosis have been characterized by in vitro studies, unveiling potential therapeutic targets for the manipulation of these regulatory pathways. In this chapter, we outline some of the key assays that are used to assess neutrophil apoptosis in vitro, together with methods to assess activation of the apoptotic machinery and phagocytic clearance of apoptotic neutrophils.

Evaluation of post-thaw CFU-GM: clinical utility and role in quality assessment of umbilical cord blood in patients receiving single unit transplant.

BACKGROUND: The CFU assay is considered the only in vitro assay that assesses the biologic function of hematopoietic stem and progenitor cells (HSPC). STUDY DESIGN AND METHODS: To investigate the impact of post-thaw CFU-GM counts on the quality of umbilical cord blood (UCB), we studied transplant outcomes in 269 patients receiving single UCB transplant. We also correlated the post-thaw CFU-GM counts of 1912 units with the pre-freeze and post-thaw graft characteristics, hoping to optimize selection criteria of UCB. Data analysis included: total nucleated cells, viability, CD34+, nucleated red blood cells (NRBC), hematocrit, frozen storage time, and cord blood bank (CBB). RESULTS: We demonstrated an association between post-thaw CFU-GM dose and the speed of neutrophil and platelet engraftment (p < 0.01). Higher post-thaw CFU-GM dose showed an increased benefit for neutrophil and platelet engraftment (p < 0.01). Post-thaw CD34+ cell dose and CFU-GM dose were strongly correlated (r = 0.78). However, CFU-GM dose showed additional benefit for patients receiving the lowest quartile of CD34+ dose. HLA disparity did not adversely impact either neutrophil or platelet engraftment. Post-thaw CFU-GM/million nucleated cells plated showed moderate correlation with pre-freeze and post-thaw CD34+ and weak correlation with other parameters. Post-thaw CFU-GM was not influenced by storage time, but was impacted by the CBB from which the unit is obtained (p < 0.01). CONCLUSION: Post-thaw CFU-GM is an effective measure of the quality and efficacy of the UCB graft, particularly adding valuable clinical information when the CD34+ cell dose is low. Consideration of pre-freeze CD34+ cell content and CBB as additional selection criteria is warranted.

Acute oral toxicity test and assessment of combined toxicity of cadmium and aflatoxin B1 in kunming mice.

Cadmium and aflatoxin B1 (AFB1) are both common and widespread pollutants in food and feed. There are several reports on toxicity induced by Cadmium or AFB1 alone, but few address the toxicity caused by co-exposure to the two substances. In this study, 42 female and 42 male Kunming (KM) mice were divided into seven groups to test the acute oral toxicity of CdCl2 and AFB1, using Karber's method. The combined toxicity was assessed using the Keplinger evaluation system. Acute toxicity symptoms, deaths, and body and organ weights were evaluated, and hematological, blood biochemical, and histopathological analyses were conducted. The results revealed the following median lethal doses (LD50): LD50(Female KM mice) = 62.56 mg/kg; LD50(Male KM mice) = 48.79 mg/kg; LD50(KM mice)=55.27 mg/kg. The combined toxicity of AFB1 and CdCl2 showed an additive effect in mice, and an increase in the mixed dose of AFB1 and CdCl2 resulted in greater toxicity. These results demonstrated that the combined toxicity of AFB1 and CdCl2 was greater than the toxicities of the individual components in mice; thus, this may cause particular challenges when addressing these hazards in food and feed and the associated risk to human and animal health.

A genome-wide association study in individuals of African ancestry reveals the importance of the Duffy-null genotype in the assessment of clozapine-related neutropenia.

Individuals of African ancestry in the United States and Europe are at increased risk of developing schizophrenia and have poorer clinical outcomes. The antipsychotic clozapine, the only licensed medication for treatment-resistant schizophrenia, is under-prescribed and has high rates of discontinuation in individuals of African ancestry, due in part to increased rates of neutropenia. The genetic basis of lower neutrophil levels in those of African ancestry has not previously been investigated in the context of clozapine treatment. We sought to identify risk alleles in the first genome-wide association study of neutrophil levels during clozapine treatment, in 552 individuals with treatment-resistant schizophrenia and robustly inferred African genetic ancestry. Two genome-wide significant loci were associated with low neutrophil counts during clozapine treatment. The most significantly associated locus was driven by rs2814778 (ß = -0.9, P = 4.21 × 10-21), a known regulatory variant in the atypical chemokine receptor 1 (ACKR1) gene. Individuals homozygous for the C allele at rs2814778 were significantly more likely to develop neutropenia and have to stop clozapine treatment (OR = 20.4, P = 3.44 × 10-7). This genotype, also termed "Duffy-null", has previously been shown to be associated with lower neutrophil levels in those of African ancestry. Our results indicate the relevance of the rs2814778 genotype for those taking clozapine and its potential as a pharmacogenetic test, dependent on the outcome of additional safety studies, to assist decision making in the initiation and on-going management of clozapine treatment.

Assessment of Neutrophil Chemotaxis Upon G-CSF Treatment of Healthy Stem Cell Donors and in Allogeneic Transplant Recipients.

Neutrophils are crucial for the human innate immunity and constitute the majority of leukocytes in circulation. Thus, blood neutrophil counts serve as a measure for the immune system's functionality. Hematological patients often have low neutrophil counts due to disease or chemotherapy. To increase neutrophil counts and thereby preventing infections in high-risk patients, recombinant G-CSF is widely used as adjunct therapy to stimulate the maturation of neutrophils. In addition, G-CSF is utilized to recruit stem cells (SCs) into the peripheral blood of SC donors. Still, the actual functionality of neutrophils resulting from G-CSF treatment remains insufficiently understood. We tested the ex vivo functionality of neutrophils isolated from blood of G-CSF-treated healthy SC donors. We quantified chemotaxis, oxidative burst, and phagocytosis before and after treatment and detected significantly reduced chemotactic activity upon G-CSF treatment. Similarly, in vitro treatment of previously untreated neutrophils with G-CSF led to reduced chemotactic activity. In addition, we revealed that this effect persists in the allogeneic SC recipients up to 4 weeks after neutrophil engraftment. Our data indicates that neutrophil quantity, as a sole measure of immunocompetence in high-risk patients should be considered cautiously as neutrophil functionality might be affected by the primary treatment.

High-dose acute exposure of paraquat induces injuries of swim bladder, gastrointestinal tract and liver via neutrophil-mediated ROS in zebrafish and their relevance for human health risk assessment.

The exact toxicological mechanisms of paraquat (PQ) poisoning are not entirely clear, especially on the high-level acute exposure. To assess the health risk of PQ, especially to suicidal individuals, accidental ingestion eaters, occupational groups, and special multitude, firstly we explored the acute toxic effect and the possible mechanisms of high-level exposure of PQ using zebrafish. The mainly target organs of PQ were swim bladder which is the homolog of the mammalian lung, followed by gastrointestinal tract and liver. Morphological malformations which were further defined by histopathologic examination include smaller size, fibrosis and inflammatory cell invasion for swim bladder; irregularly arranged or dissolved epithelial folds, loss of villous architecture, and ecclasis of mucosal cells in a smaller lumen for gastrointestinal tract; as well as smaller size, degeneration, fibrous proliferation, atrophy for liver. In addition, PQ enhanced leukocyte recruitment (neutrophil migrated first, followed by macrophage) into swim bladder and induced ROS which can be scavenged by glutathione. Moreover, qRT-PCR results showed that PQ increased the expression level of genes involved in the inflammatory response, such as L-1ß, IL-6, IL-8, TNF-α, TNF-ß, IFN-1, TGF-ß, and NF-kB. For the first time, our results demonstrated that acute exposure of PQ induced pulmonary toxicity which was followed by gastrointestinal and hepatic toxicity via neutrophil-mediated ROS in zebrafish. In summary, these findings generated here will contribute to our better understanding of characteristics of PQ acute poisoning and can provide valuable information on better PQ poisoning treatments, occupational disease prevention, and providing theoretical foundation for risk management measures.

Poisson pre-processing of nonstationary photonic signals: Signals with equality between mean and variance.

Photonic signals are broadly exploited in communication and sensing and they typically exhibit Poisson-like statistics. In a common scenario where the intensity of the photonic signals is low and one needs to remove a nonstationary trend of the signals for any further analysis, one faces an obstacle: due to the dependence between the mean and variance typical for a Poisson-like process, information about the trend remains in the variance even after the trend has been subtracted, possibly yielding artifactual results in further analyses. Commonly available detrending or normalizing methods cannot cope with this issue. To alleviate this issue we developed a suitable pre-processing method for the signals that originate from a Poisson-like process. In this paper, a Poisson pre-processing method for nonstationary time series with Poisson distribution is developed and tested on computer-generated model data and experimental data of chemiluminescence from human neutrophils and mung seeds. The presented method transforms a nonstationary Poisson signal into a stationary signal with a Poisson distribution while preserving the type of photocount distribution and phase-space structure of the signal. The importance of the suggested pre-processing method is shown in Fano factor and Hurst exponent analysis of both computer-generated model signals and experimental photonic signals. It is demonstrated that our pre-processing method is superior to standard detrending-based methods whenever further signal analysis is sensitive to variance of the signal.

Identification of LukPQ, a novel, equid-adapted leukocidin of Staphylococcus aureus.

Bicomponent pore-forming leukocidins are a family of potent toxins secreted by Staphylococcus aureus, which target white blood cells preferentially and consist of an S- and an F-component. The S-component recognizes a receptor on the host cell, enabling high-affinity binding to the cell surface, after which the toxins form a pore that penetrates the cell lipid bilayer. Until now, six different leukocidins have been described, some of which are host and cell specific. Here, we identify and characterise a novel S. aureus leukocidin; LukPQ. LukPQ is encoded on a 45 kb prophage (ΦSaeq1) found in six different clonal lineages, almost exclusively in strains cultured from equids. We show that LukPQ is a potent and specific killer of equine neutrophils and identify equine-CXCRA and CXCR2 as its target receptors. Although the S-component (LukP) is highly similar to the S-component of LukED, the species specificity of LukPQ and LukED differs. By forming non-canonical toxin pairs, we identify that the F-component contributes to the observed host tropism of LukPQ, thereby challenging the current paradigm that leukocidin specificity is driven solely by the S-component.

The Kidd (JK) Blood Group System.

The Kidd blood group system was discovered in 1951 and is composed of 2 antithetical antigens, Jka and Jkb, along with a third high-incidence antigen, Jk3. The Jk3 antigen is expressed in all individuals except those with the rare Kidd-null phenotype. Four Kidd phenotypes are therefore possible: Jk(a+b-), Jk(a-b+), Jk(a+b+), and Jk(a-b-). The glycoprotein carrying the Kidd antigens is a 43-kDa, 389-amino acid protein with 10 membrane-spanning domains which functions as a urea transporter on endothelial cells of the renal vasa recta as well as erythrocytes. The HUT11/UT-B/JK (SLC14A1) gene encoding this glycoprotein is located on chromosome 18q12-q21. The Jka and Jkb antigens are the result of a single-nucleotide polymorphism present at nucleotide 838 resulting in an aspartate or asparagine amino acid at position 280, respectively. The Kidd blood group can create several difficult transfusion situations. Besides the typical acute hemolytic transfusion reactions common to all clinically relevant blood group antigens, the Kidd antigens are notorious for causing delayed hemolytic transfusion reactions due to the strong anamnestic response exhibited by antibodies directed against Kidd antigens. The Kidd-null phenotype is extremely rare in most ethnic groups, but is clinically significant due to the ability of those with the Kidd-null phenotype to produce antibodies directed against the high-incidence Jk3 antigen. Anti-Jk3 antibodies behave in concordance with anti-Jka or anti-Jkb possessing the capability to cause both acute and delayed hemolytic reactions. Antibodies against any of the 3 Kidd antigens can also be a cause of hemolytic disease of the fetus and newborn, although this is generally mild. In this review, we will outline the makeup of the Kidd system from its historical discovery to the details of the Kidd gene and glycoprotein, and then discuss the practical aspects of Kidd antibodies and transfusion reactions with an extended focus on the Kidd-null phenotype. We will end with a brief discussion of the donor aspects related to the screening and supply management of blood from donors with the rare Jk(a-b-) phenotype.