P-selectin expression. Shown is P-selectin expression on platelets relative to P-selectin on HUVEC. Platelets were incubated on collagen, as described in the Materials and Methods, and an ELISA was performed to measure cell-surface P-selectin expression. HUVEC were stimulated with histamine (25 μmol/L) for 10 minutes, and then the P-selectin expression was measured. (▨) Nonspecific binding; (▪) P-selectin expression.