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Lack of cross-reactivity between anti-A IgG isoagglutinins and anti-SARS-CoV-2 IgG antibodies

  • Massimo Franchini , Paola Moi , Marzia Cortellazzi , Nadia Danese , Stefania Caruso , Paola Pasolini , Sabina Ferrazzo , Stefania Piccinini , Anna Dall’Oglio , Paola Zovetti , Nadia Negri , Donatella Braga , Cristina Pasquali , Elisa Zuliani and Claudia Glingani
Published/Copyright: February 10, 2021

To the Editor,

The ABO blood group is the most important among human blood group systems and consists of complex carbohydrate moieties at the extracellular surface of red blood cell (RBC) membrane [1]. Along with their expression on RBCs, ABO antigens (namely, A, B, AB, and O) are also highly expressed on the surface of a variety of human cells and tissues [2]. Although the physiologic role of ABO antigens and their related anti-A and anti-B natural isoagglutinins is still largely unknown, they play a prominent role in blood transfusion and cell, tissue, and organ transplantation [2]. In addition, several studies have documented over the last 50 years a close link between ABO blood groups and a wide array of diseases, including cancers and cardiovascular disorders [3]. The latter association is particularly relevant, considering the profound influence of ABO antigens on hemostasis, particularly in modulating von Willebrand factor (WVF) and factor VIII (FVIII) circulating levels [4]. Also the ABO blood group-related susceptibility to various types of viral infections, including HIV, hepatitis B, dengue and influenza viruses, has been consistently reported by several investigators over the last 20 years [5]. This issue has recently gained a renewed interest thanks to the first observations on the association between ABO blood type and Coronavirus Disease 2019 (COVID-19) from China, where the infection began and quickly spread around the world [6]. In particular, it has been hypothesized that individuals belonging to O blood type are less susceptible to Severe Acute Respiratory Syndrome Coronavirus 2 (SARS‐CoV‐2) infection than those belonging to non‐O blood groups or that have a milder disease [7]. The hypothesized reason for this phenomenon lies in the presence in O blood group subjects of anti‐A isoagglutinins, predominantly of IgG class, which would prevent the binding of SARS‐CoV‐2 to its angiotensin converting enzyme (ACE)-2 receptor thereby inhibiting the virus entry into the targeted human cells [8]. Given such association, in this study we wanted to check if there is a cross-reactivity between anti-A IgG isoagglutinins and anti-SARS-CoV-2 IgG antibodies in patients recovered from COVID-19 selected for convalescent plasma donation.

Serum samples from 12 consecutive convalescent plasma donors belonging to blood group O (group I-cases) were collected and anti-A IgG antibodies were titrated according to standard technique (dilution phosphate-buffered saline-treated control serum and dithiothreitol-treated serum) using a micro-column technology (Ortho Clinical Diagnostics, Raritan, NJ, USA) [9]. All these samples were also titrated for anti-SARS-CoV-2 neutralizing antibodies using the plaque reduction neutralization test (PRNT), as previously described [10]. In addition, the chemiluminescent immunoassay (CLIA)-based technology (LIAISON SARS-CoV-2 S1/S2 IgG, DiaSorin, Vercelli, Italy) was used on these serum samples for the quantitative determination of anti-SARS-CoV-2 IgG antibodies, which were measured at basal and post-adsorption (incubation for 60 min at 37 °C) on blood group A RBCs, to remove from convalescent plasma donors’ serum anti-A IgG antibodies [10]. All these tests were performed also in a control group (group II) of 12 consecutive O blood type convalescent plasma donors, with the only difference that their serum was adsorbed on O blood type RBCs for the measurement of post-RBC adsorption anti-SARS-CoV-2 IgG antibodies.

The two groups (group I-cases and group II-controls) of convalescent plasma donors were comparable in terms of male-female ratio (5.0 vs. 3.3) and median age (46.5 vs. 46.0 years). The median anti-A IgG isoagglutinin titer and anti-SARS-CoV-2 neutralizing titer in the study group (group I) were 96 (range 32–256) and 80 (range 20–320), respectively. The median anti-A IgG isoagglutinin titer and anti-SARS-CoV-2 neutralizing titer in the control group (group II) were 80 (range 32–256) and 120 (range 20–320), respectively. No significant differences were observed in these parameters (i.e., anti-A IgG isoagglutinin titer and anti-SARS-CoV-2 neutralizing titer) between the two groups. Regarding the anti-SARS-CoV-2 IgG CLIA-based assay, the median basal and post-RBC (blood group A) serum adsorption levels in group I were 82 UA/mL (range 8–249 UA/mL) and 73 UA/mL (range 7–240 UA/mL), respectively. In group II (control group), the median basal and post-RBC (blood group O) anti-SARS-CoV-2 IgG levels were 95 UA/mL (range 33–265 UA/mL), and 81 UA/mL (range 22–254 UA/mL), respectively (Table 1). Using Student’s t test for statistical analysis, no statistically significant difference between basal and post-RBC adsorption anti-SARS-CoV-2 IgG levels were detected in both group I and group II convalescent plasma donors. No difference was also observed comparing the same parameters between the two groups. A similar difference between median anti-SARS-CoV-2 IgG levels pre- and post-RBC adsorption studies was observed in group I and group II (11 and 15%, respectively). Thus, considering that in group I the donors’ serum was adsorbed on A blood type RBCs and in group II it was adsorbed on O blood type RBCs, this slight difference was certainly not due to the presence of anti-A IgG isoagglutinins but rather to the adsorption procedure itself.

Table 1:

Relationship between anti-A IgG isoagglutinins and anti-SARS-CoV-2 IgG antibodies in cases and controls.

Convalescent plasma donorsProgressive no.SexAge, yearsAnti-A IgG isoagglutinin titerAnti-SARS-CoV-2 neutralizing titerAnti-SARS-CoV-2 IgG antibody titer
BasalPost-RBC adsorptionap-Valueb
O blood type (Group I, cases)1Male6032320221201NS
2Female44321608173
3Male3032204034
4Male47321608373
5Male29128802016
6Male4612840130123
7Male5625680171152
8Male4464160249240
9Male64128160132117
10Male37642087
11Female57256406354
12Male50128203026
O blood type (Group II, controls)1Male30321607450NS
2Male4564203322
3Female44128206954
4Male54648011090
5Male57641608072
6Female30256160240220
7Female3232320265254
8Male61256160223201
9Male6312880174152
10Male47128160228221
11Male54128406555
12Male28128803523
  1. SARS-CoV-2, Severe Acute Respiratory Syndrome-Coronavirus-2; RBC, red blood cell, NS, not significant. aPost-adsorption of donors’ serum on A blood type RBCs (group I-cases) or O blood type RBCs (group II-controls). bStudent’s t test.

In conclusion, in this study we demonstrated the lack of cross-reactivity between anti-A IgG isoagglutinins and anti-SARS-CoV-2 IgG antibodies measured using an immunoassay, therefore excluding a possible interference of isoagglutinins on seroneutralizing tests.


Corresponding author: Massimo Franchini, MD, Department of Hematology and Transfusion Medicine, Carlo Poma Hospital, Mantova, Italy, Phone: +39 0376 201234, Fax: +39 0376 220144, E-mail:

  1. Research funding: None declared.

  2. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  3. Competing interests: Authors state no conflict of interest.

  4. Informed consent: Informed consent was obtained from all individuals included in this study.

  5. Ethical approval: The local Institutional Review Board deemed the study exempt from review.

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Received: 2021-01-07
Accepted: 2021-01-25
Published Online: 2021-02-10
Published in Print: 2021-06-25

© 2021 Walter de Gruyter GmbH, Berlin/Boston

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