The inhibitory capacity of the sera was tested prior to IgG purification, since crude purification of IgG may introduce a bias due to differential efficacy of purification of different antibody isotypes. Only FV2 and DBL2X specific rabbit serum appeared to inhibit more than the negative control rabbit serum ( Figure 3A ). This was in contrast to the responses measured using FCM against the native protein in DBL5e and DBL6e specific rabbit sera ( Figure 2C ), where high reactivity was found. The inhibitory capacity of rat serum was also not determined by the surface reactivity, for instance there was prominent inhibition using DBL4e and FV2 specific serum and lack of inhibition by DBL6e specific antibodies ( Figure 3A ), although DBL6e specific antibodies also were highly reactive against native VAR2CSA ( Figure 2B ). To rule out that IgM or unspecific effects of serum did not mediate the inhibitory capacity, we purified IgG from both inhibitory and noninhibitory sera and tested them in inhibition of binding assays at 0.5 mg/ml. The level of inhibition was essentially sustained in the IgG preparations ( Figure 3B&C ).