This study characterized a human monoclonal antibody against the major grass pollen allergen Phl p 5 and found its inferred unmutated common ancestor already binds with low-nanomolar affinity, showing high affinity can be reached with minimal somatic hypermutation. A single somatic mutation left binding intact but reduced thermostability and increased the antibody's hydrodynamic radius. Flow Induced Dispersion Analysis (FIDA), on a Fida Neo, was used to measure the hydrodynamic radius and monodispersity of the antibody variants, while binding affinity itself was measured by surface plasmon resonance. It is an example of using FIDA to measure antibody size, monodispersity and stability in solution.