Since the Austrian physiologist Carl Landsteiner discovered the ABO blood type in 1901, scientists have continued to study the theory of blood types for more than 100 years. With the development of technology, it has been found that human blood group has at least more than 30 systems such as ABO group, Rh group, HLA and Lewis, which has important application in clinical transfusion and organ transplantation . More and more studies have found that the ABO blood group antigen system is closely related to human microbial infections such as bacteria, viruses, and parasites . Researchers have presented findings showing that natural antibodies to human red blood cell type B antigens can directly react with some E. coli strains; therefore in a large-scale retrospective report demonstrating that patients with type B and AB have a higher risk of E. coli-related sepsis than those with type O and A. At the same time, research on the relationship between ABO blood group and malaria also found that the prognosis of patients with blood type O was significantly better than that of patients with blood type A. In addition, the research related to ABO blood group antigen and virus infection is also very extensive, such as Norovirus, E-B virus, HBV, HIV, influenza virus[5–10]. Most common respiratory virus infections are characterized by family clustering, which may be related to the transmission mechanism of the virus itself, but it also suggests that genetic factors may affect the disease. In 2003, Hong Kong scholars found that patients with SARS infection without protective measures were mostly individuals with non-O antigen when the SARS epidemic broke out; on the contrary, people with type O had better resistance to the virus. In short, diversity of ABO antigen causes significant differences in host susceptibility to pathogens, yet the mechanism is still not completely clear.
Human ABO blood group antigens are divided into four types: A, B, AB, and O, the difference among them lies in the glycosyl structure of the antigen terminal which are determined by the three genes A, B, and O locating at the end of chromosome 9. Among them, gene A and B are dominant alleles with the same advantages and encode different glycosyltransferases, whereas O gene allele acting as a stealth gene does not have the ability to encode functional enzymes. Except for red blood cells, ABO antigen it is expressed in epithelial cells, vascular endothelial cells, platelets and other parts, moreover, these antigens can be distributed in a variety of body fluids in free form.
In recent years, studies have found that blood group antigens also play an important role in the immune system. For example, Blood A antigen can directly act as a receptor for rotavirus subtype HAL1166, and the pathogenic ability of the subtype may also be related to the strength of the binding to type A antigen. In addition, type O plasma significantly inhibits the binding of SARS-COV spinous protein to host cell receptor ACE2, and this inhibitory effect has a clear dose-dependent relationship. These suggest that the ABO blood group antigen system has an important role in the process of virus invasion into the human body, for the reason that blood group antigens are likely to cause large differences in individual sensitivity to pathogens.
Of late, the outbreak of 2019 new coronavirus pneumonia (COVID-19) which was characterized as a pandemic by the World Health Organization (WHO) have occurred in many countries around the world, but we still know little about the pathogenesis of the disease. In order to explore the susceptibility factors for the occurrence of COVID-19, we analyzed the association between the blood type of the patients diagnosed with COVID-19 and the asymptomatic carriers of SARS-COV-2 to the occurrence of the disease.