If a serum with cross-reactive antibody to the birch allergen Bet v 1 is incubated with Bet v 1 on the solid-phase, the binding curve is different in two ways: 1) it rises to a higher level and 2) it is shifted to the left compared to the binding observed with the cross-reactive apple allergen Mal d 1 on the solid phase. The first observation reflects that only a fraction of the epitopes is cross-reactive (in this example: 50%, as indicated by the vertical arrow). The second observation reflects that only a fraction of the antibodies is cross-reactive, but also that the affinity is usually lower for the cross-reactive allergen (Mal d 1) compared to the sensitising allergen (Bet v 1). In these model calculations, the concentration of Bet v 1 epitopes is set at 1; 50% of these epitopes are assumed have a cross-reactive homologue in Mal d 1; 40% of the IgE antibodies are assumed to be cross-reactive. The upper curve represents binding to Bet v 1, assuming a dissociation constant KD equal to 1. The next 3 curves represent binding to Mal d 1 at decreasing affinities (KD equal to 1, 5 and 25, respectively. Note that this type of experiment does not prove cross-reactivity. The observed binding could in theory also be due to co-sensitization. This can be investigated by inhibition assays (see figure 3).