How does epitope density affect apparent avidity?

Why

Monovalent binding is influenced by intrinsic kinetic rates, while multivalent engagement adds geometric opportunities for rebinding. On a dense antigen surface, an IgG may retain one arm while the other reattaches, producing a slow apparent off-rate. On a sparse target, geometry may force predominantly single-arm binding.

Design a controlled density series using comparable cells or engineered surfaces and keep the antibody format, antigen presentation and incubation time fixed. Compare intact IgG with Fab fragments to separate valency from changes in epitope accessibility. Watch for mass-transport effects in SPR or biosensor assays, which can imitate slower kinetics.

For example, an engineered high-expression cell line can overestimate binding strength relevant to a low-expression tumor.