Neutrophils are abundant in inflamed joints of patients with RA. The amount (A) and percentage (B) of neutrophils (CD11b+CD15+) in synovial fluid (SF) from RA and osteoarthritis (OA) patients detected by FCS. (C) Representative photomicrographs showing immunohistochemical staining of OA and RA synovial tissues with neutrophil marker anti-CD15 Ab. Arrow points to representative neutrophils (red). Original magnification × 400. (D) Neutrophil infiltration in joint tissue sections detected by FCS. Isolation and staining of cells from ST as described in Methods. Data are representative of at least three independent experiments. * P <0.05, ** P <0.01. Ab, antibody; FCS, flow cytometry sorting; RA, rheumatoid arthritis.