Immunoblot analysis of cytoplasmic extracts from PLC/PRF/5 cells cultured in the presence of different concentrations of human monoclonal anti-HBs. The cells were cultured in medium with 0.2 mg of anti-HBs/ml (lanes 1), with 1.0 mg of anti-HBs/ml (lanes 2), with 2.0 mg of anti-HBs/ml (lanes 3), or without human IgG (lanes 4). (a) Amido black staining of proteins. (b) Detection of human IgG. (c) Detection of HBsAg. (d) Detection of FcRn. INT, intensity of the chemiluminescence signal measured by FluorS MultiImager in arbitrary units. The arrows indicate the signals for the γ chain of IgG (50 kDa), for both forms of HBsAg (24 and 27 kDa), and for the heavy chain of FcRn (45 kDa). LM, protein length marker.