The pro-aggregatory strain of H pylori stimulates the expression of P-selectin. Platelets were fixed with formaldehyde and labeled with anti-CD41 IgG and P-selectin, as well as the anti-CD62P antibody. They were then analyzed using flow cytometry. (A) No aggregation occurred in response to LPSs extracted from H pylori. These were therefore used as negative controls. (B) P-selectin activation (platelet/bacteria ratio = 100:1) was found to occur only in response to pro-aggregatory Hp 49503. Hp 43504, Hp 51932, or any LPSs extracted from H pylori did not induce P-selectin expression. (C) The activation was found to occur in a concentration-dependent manner in pro-aggregatory Hp 49503 infection. (D) A significant increase in P-selectin expression was observed only in Hp IgG⁺ in response to H pylori infection.