We demonstrate a microfluidic method for ultra-sensitive protein detection in serum. First, ‘large’ (2.8 μm) antibody-functionalized magnetic beads specifically capture antigen from a serum matrix under active microfluidic mixing. Subsequently, the large beads loaded with the antigens are gently exposed to a surface pattern of fixed ‘small’ (1.0 μm) antibody-coated magnetic beads. During the exposure, attractive magnetic bead dipole–dipole interactions improve the contact between the two bead types and help the antigen-antibody immunocomplex formation, while non-specific large bead adsorption is limited by exploiting viscous drag forces in the microfluidic channel on the small-bead pattern. This efficient antigen-antibody recognition and binding mechanism mimics a biological process of selective recognition of tissue molecules, like is the case when leukocytes roll and slow down on blood vessel walls by selectin-mediated adhesion. After exposure of the large beads to the pattern of small beads, the antigen concentration is detected by simply counting the number of surface pattern-bound large magnetic beads. The new technique allows detection of proteins down to the sub-zeptomole range. In particular, we demonstrate detection of only 200 molecules of Tumor Necrosis Factor-α (TNF-α) in a serum sample volume of 5 μL, corresponding to a concentration of 60 attomolar or 1 fg/mL.