demonstrated that this adhesion of SARS\CoV spike protein to ACE2 could be inhibited by natural anti\A antibodies. 88 The same can be extended to SARS\CoV\2. 89 Thus, one might assume that both blood group O and B should provide protection, while numerous studies revealed that blood group B do not alter the risk of acquiring COVID\19. 38 , 49 , 90 For instance, Gerard et?al. SARS\CoV\2 contamination and developing severe outcomes. Our review shows that blood group O protects individuals against SARS\CoV\2, whereas blood group A predisposes them to being infected. Although the association between blood groups and outcomes of COVID\19 is not consistent, it is speculated that non\O blood group carriers with COVID\19 are at higher risk of developing severe outcomes in comparison to O blood group. The conversation between blood groups and SARS\CoV\2 contamination is hypothesized to be as result of natural antibodies against blood group antigens that may act as a part of innate immune response to neutralize viral particles. Alternatively, blood group antigens could serve as additional receptors for the computer virus and individuals who are capable of expressing these antigens on epithelial cells, which are known as secretors, would then have a high propensity to be affected by SARS\CoV\2. contamination than those with blood group O. 27 Moreover, the efficacy of infectious disease related vaccines may be influenced by blood group distribution in the target populace. 28 , 29 The mechanism in which blood group antigens may confer susceptibility or protection from infectious brokers or influence the evolution of diseases have yet to be elucidated. However, there is some underlying evidence to suggest that blood group antigens may play a key role as receptors and/or cofactors for several infectious Eng brokers 11 , 30 including Norwalk computer virus and which interact with ABO antigens Ardisiacrispin A to successfully bind with gastric mucosa. 31 , 32 Furthermore, various pathogens are capable of expressing blood group antigens identical or cross\reactive epitopes on their surfaces. 11 Therefore, natural antibodies against ABO antigens may act as a part of innate immunity that can attenuate contamination. Ardisiacrispin A 11 The next general blood group category is the Rhesus (Rh) system, determined by the presence or absence of Rh or D antigen. Unlike the ABO system, Rh phenotypes are associated with few diseases, most of which are haemolytic diseases of newborns that occur as a consequence of Rh mismatching between mother and offspring. 33 4.?BLOOD GROUPS AND RISK OF CONTRACTING WITH SARS\CoV\2 The initial idea of a relationship between blood groups and coronavirus infections refers to 2005, where Cheng et?al. examined the association of ABO blood groups and risk Ardisiacrispin A of SARS\CoV contamination on 45 health care staff who were not guarded by relevant gear in exposure to affected patients. 34 The comparison revealed that individuals with blood group O had a lower risk of contamination compared to non\O blood groups (odds ratio [OR]?=?0.18; 95% confidence interval [CI]: 0.04C0.81; gene, encoding fucosyltransferase 2 enzyme. 65 Based on synthesis of this enzyme, approximately 80% of the population are secretor and 20% are non\secretors. 65 Expression of blood group antigens on mucosal cells have raised some questions about whether the Ardisiacrispin A interactions with infectious brokers play a role on entry processes of pathogens. In this case, it was shown that in non\secretors, lack of blood type antigen expression on mucosal cells offers some degree of protection from several pathogens, which then potentially bind to these antigens on mucosal surfaces. 65 , 66 Ardisiacrispin A , 67 Recently, it has been shown that this receptor binding domain name (RBD) of SARS\CoV\2 spike protein exhibited only low\level affinity for binding to A, B, and H antigens on RBCs but, when exposed to blood group antigens expressed on respiratory epithelial cells, a high affinity was found toward binding to blood group A antigen in comparison with B and H antigens. 68 Consistently, Valenti et?al. decided that in non\O blood group patients affected by SARS\CoV\2, non\secretor phenotype was significantly associated with reduced need of mechanical ventilation and intensive care unit (ICU) admission as compared to secretor phenotypes (OR?=?0.57; 95% CI: 0.37C0.87; gene and COVID\19 prevalence nor mortality. 70 Thus, clarifying the exact role of secretor status on SARS\CoV\2 contamination requires further investigations. 6.?BLOOD GROUPS AND PROGNOSIS OF COVID\19 The majority of studies concluded that ABO blood groups cannot influence the outcomes of COVID\19 and are not associated with mortality. 17 , 39 , 41 , 50 , 51 , 60 ,.