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In this paper we have extended our earlier studies of the action of increasing Factor I concentration on complement activation by using a soluble activator, lipopolysaccharide (LPS) endotoxin, and using human erythrocytes as a source of CR1 – the co-factor needed for the final clip of iC3b to C3dg by Factor I. Using this more physiological system, the results show that we can predict that a quite modest increase in Factor I concentration – 22 μg/ml of extra Factor I – will convert the activity of the highest risk sera to those of the lowest risk. Preliminary experiments have been performed with erythrocytes allotyped for CR1 number. While we have not been able to perform an adequate study of their co-factor activities in our assays, preliminary experiments suggest that when Factor I levels are increased the difference produced by different allotypes of red cells is largely overcome. This suggests that in patients with paroxysmal nocturnal haemoglobinuria (PNH) treated with eculizumab, additional treatment with Factor I may be very useful in reducing the need for blood transfusion. We have also explored the age-related allele frequency for the two polymorphisms of Factor H and the polymorphism of C3. In our population, unlike the 1975 study, we found no age variation in the allele frequency in these polymorphisms. This may, however, reflect that the Cambridge BioResource volunteers do not include many very young or very elderly patients, and in general comprise a population not greatly at risk of death from infectious disease.