Detailed procedures have been described previously (13 (link), 19 (link), 20 (link)). In brief, human RBCs were isolated from the whole blood of healthy volunteers according to a protocol approved by the Institutional Review Board of the Georgia Institute of Technology. RBCs were biotinylated, conjugated with streptavidin, and incubated with biotinylated pMHC for adhesion frequency assay with a micropipette. In the case of BFP experiments, silanized beads were covalently linked to streptavidin-maleimide (Sigma-Aldrich) then conjugated with subsaturating biotinylated pMHC.