Immunohistochemical analysis of human biopsies. Tissue biopsies were obtained from patients with necrotizing fasciitis caused by S pyogenes (top) and from healthy volunteers (bottom). The biopsies were sectioned and immunohistochemically stained for streptococcal M1 protein, fXIII, and N-ϵ-γ-glutamyl-lysine. Stainings without primary antibodies were negative (data not shown). The scale bars indicate 50 μm.