To test whether the reactivity against the native protein on the IE surface was associated with the apparent antigenicity of the recombinant domains, as measured by ELISA, the mean fluorescence values (MFI) was correlated to the EC50 titer values for all single domains. There was a highly significant linear correlation for mice and rabbits (data not shown) (r = 0.847, P,0.0001 & r = 0.809, P = 0.0008, respectively, Pearson), whereas the linear dependency for rats was somewhat lower (data not shown) (r = 0.476, P = 0.01, Pearson).