We introduce a new modeling approach for understanding the fundamental principle of bead-based surface coverage immunoassays that rely on the specific interaction between surface-bound “small” magnetic beads (1 μm in diameter) and a flow of “large” magnetic beads (3 μm in diameter). The latter ones carry target antigens and are simultaneously used as detection labels. This immunoassay technique allows attaining extremely low (attomolar) limits of detection, with strongly sub-linear dose-response curves, but the origin of these features remained unclear up to now. Our comprehensive theoretical model allows complete understanding of these unsolved issues.