 Solution

In surface plasmon resonance, target protein in the mobile phase passes over the immobilized antibody domain. Binding changes the optical response near the sensor surface; response increases during association and falls when unbound target is washed away. The successive injections give larger responses as the supplied target concentration increases.

For a simple 1:1 model,The reported fit gives $K_D=78\mathrm{nM}$. This is a binding estimate for the tested preparation and model. Surface heterogeneity, transport limitations or rebinding could cause deviations; the visible mismatch between parts of the trace and fit discourages treating the binary model as exact.