To examine the antigenicity of the recombinant proteins the levels of antigen-specific IgG from immunized animals were measured using ELISA. All animal species immunized with individual DBL-domains or FV2 produced an IgG response, with a typical sigmoid shaped dose response curve of the antibody titrations ( Figure 1 ). However, the levels of antibodies to each of the DBL-domains as well as to FV2 differed depending on the animal species used (for all statistical analysis of antigenic differences see Table S2 ). In mice, the DBL6e immunizations resulted in the most potent IgG response with a significantly higher titre than that of FV2 and the other DBL-domains. Following DBL6e in potency was DBL5e and thereafter a lower response induced by DBL1X, DBL2X, DBL3X, DBL4e and FV2 of similar potency. As observed in mice, immunizations of rats with DBL6e resulted in a significantly higher IgG response compared to all other immunogens. DBL5e and FV2 induced in rats a more potent response than DBL1X, DBL2X, and DBL3X ( Figure 1B ) but also DBL4e induced a high antibody response. In rabbits, the seven tested antigens produced IgG responses that can be grouped in two according to the levels of responses ( Figure 1C ). The DBL6e, DBL5e and FV2 produced a high antibody response whereas IgG responses to DBL1X, DBL2X, DBL3X and DBL4e domains were lower and essentially similar.