Synovial distribution pattern of 5-Lipoxygenase. CD163+ macrophages, CD66b+ neutrophils and tryptase-positive mast cells express 5-lipoxygenase (5-LO) in rheumatoid arthritis synovium. Photomicrographs showing fluorescent staining of (a) CD163+ cells, (b) CD66b+ cells and (c) tryptase-positive cells (Alexa 546, red), 5-LO-positive cells (Alexa 488, green), and superimposed staining. White arrows, double-positive mast cells expressing 5-LO. Original magnification: ×400.