Qua nghiên cứu cho thấy những người có máu nhóm O có thể có nguy cơ ít hơn để phát thành biến chứng nặng do sốt rét P.falciparum, một nghiên cứu được tiến hành bởi các nhà nghiên cứu ở Anh, Mỹ, Mali và Kenya cho biết. Một nghiên cứu bệnh chứng ghép đôi trên 567 trẻ em người Mali cho thấy nhóm máu O xuất hiện chỉ chiếm 21% trong tổng số trường hợp sốt rét nặng so với 44–45% trong số những ca sốt rét không biến chứng ở nhóm chứng và nhóm chứng những người khỏe mạnh. Nhóm máu O có liên quan đến việc làm giảm 66% tỷ lệ phát triển thành sốt rét nặng so với nhóm bệnh nhân không phải là nhóm máu O (non-O blood groups). Những hồng cầu nhóm máu O gây cản trở sự thành lập thể hoa hồng (rosettes), nhờ đó mà giảm nguy cơ kết vón trong thành mạch máu và gây nên biến chứng trong sốt rét do P.falciparum.
P. falciparum Culture. After removal of lymphocytes (46), the Malian samples were washed and then frozen in glycerolyte to –80°C and shipped to Edinburgh. The isolates were thawed and cultured for 18–36 h as described (46) to allow maturation from ring stage to the pigmented trophozoite stage at which rosetting occurs. No additional erythrocytes were added during this short culture step. Parasite maturity was monitored by Giemsa smear, and only those with normal morphology that matured to the pigmented trophozoite stage were assessed for rosetting. Of a possible 378 P. falciparum-infected samples, 272 were put into culture, the others having been lost during a freezer malfunction. Of those cultured, 13 did not grow, and 51 were excluded because the parasitemia was too low to assess rosetting (<0.5%). Therefore, rosette frequency was assessed for 208 samples (91 uncomplicated malaria, 40 nonsevere hyperparasitemia, and 77 severe malaria).
Rosetting Assays. Rosette frequency was assessed in the first cycle of in vitro growth. An aliquot of culture suspension was stained with 25 µg/ml ethidium bromide for 5 min at 37°C. A wet preparation of the suspension (2% hematocrit) was viewed with a fluorescence microscope, and the number of mature infected erythrocytes binding two or more uninfected erythrocytes was counted. The rosette frequency is the percentage of infected erythrocytes in rosettes of 200 infected erythrocytes counted.
Malaria has been a major selective force on the human population, and several erythrocyte polymorphisms have evolved that confer resistance to severe malaria. Plasmodium falciparum rosetting, a parasite virulence phenotype associated with severe malaria, is reduced in blood group O erythrocytes compared with groups A, B, and AB, but the contribution of the ABO blood group system to protection against severe malaria has received little attention. We hypothesized that blood group O may confer resistance to severe falciparum malaria through the mechanism of reduced rosetting. In a matched case-control study of 567 Malian children, we found that group O was present in only 21% of severe malaria cases compared with 44–45% of uncomplicated malaria controls and healthy controls. Group O was associated with a 66% reduction in the odds of developing severe malaria compared with the non-O blood groups (odds ratio 0.34, 95% confidence interval 0.19–0.61, P < 0.0005, severe cases versus uncomplicated malaria controls). In the same sample set, P. falciparum rosetting was reduced in parasite isolates from group O children compared with isolates from the non-O blood groups (P = 0.003, Kruskal–Wallis test). Statistical analysis indicated a significant interaction between host ABO blood group and parasite rosette frequency that supports the hypothesis that the protective effect of group O operates through the mechanism of reduced P. falciparum rosetting. This work provides insights into malaria pathogenesis and suggests that the selective pressure imposed by malaria may contribute to the variable global distribution of ABO blood groups in the human population.