= 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,
$$
\frac{dR}{dt}=k_{\rm on}C(R_{\max}-R)-k_{\rm off}R,\qquad K_D=\frac{k_{\rm off}}{k_{\rm on}}.
$$
The reported fit gives \b[$K_D=78\,\mathrm{nM}$]. 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.
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