Characterization of human PBMCs immortalized by infection of M fermentans. Human PBMCs (K4420-Mi) that continued to proliferate for more than 5 months after infection with M fermentans (Mi) were centrifuged onto slides by cytospin for immunocytochemistry study. Practically, all cells show intensely positive staining for LCA (A) and antigen L26 (CD20, B) and negative staining for CD3 marker (C). Cells are found positive for monotypic immunostaining for lambda light chain (D) and positive for EBV latent membrane protein (LMP1, F). These cells were negative for kappa light chain staining (E). Images were captured on an Olympus BH-2 microscope (Olympus) equipped with an Olympus 35-mm film camera PM-C35DX. Original magnification was × 200 under a Splan apochromatic objective lens, numerical aperture 0.46.