What is the difference between affinity and avidity?

Why

For a simple monovalent interaction, affinity is often summarized with KD and kinetic on/off rates. An IgG can use two arms to bind epitopes on a cell, reducing apparent dissociation because one arm can remain engaged while the other rebinds. That multivalent advantage depends on epitope spacing, flexibility, receptor density, assay format and steric access.

A high apparent cell binding signal is therefore not direct evidence of higher intrinsic Fab affinity. Compare monovalent Fab and intact IgG where practical, collect kinetic binding data on well-controlled surfaces, and titrate antigen density. A bivalent antibody might appear stronger on a high-density cell line but lose much of its advantage on cells expressing few receptors.

Distinguish KD from apparent EC50 and report the molecular format used.