We carried out a study of adenosine deaminase (ADA) and MN blood group genetic polymorphisms in relation to past malarial morbidity in Sardinia and in relation to susceptibility to allergic asthma (a Th2 disorder) and Crohn's disease (a Th1 disorder) in the population of Rome. Eight hundred and eight schoolchildren, aged 7--14 years from 14 Sardinian villages located in the central area of the island, were considered. One hundred and twenty-two children with allergic asthma and 39 adult patients with Crohn's disease from the population of Rome were also studied. The data suggest an interaction between the two systems concerning resistance/susceptibility, both to malaria and to the diseases considered. In Sardinia, the frequency of the *L(M)/ADA*2 gametic type is negatively correlated with past malarial endemia, suggesting an increased susceptibility to malaria leading to its decrease in areas with high malarial endemia. In Rome, this gametic type is correlated negatively to allergic asthma and positively to Crohn's disease, suggesting a protective effect against allergic asthma and increased susceptibility to Crohn's disease.
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