Two-color immunofluorescence confocal microscopy analysis of FL expression in T lymphocytes. PBMC were settled on the slides, fixed with paraformaldehyde, permeabilized with 0.1% saponin, and stained with anti-FL MoAb M5 followed by FITC-labeled goat antirat IgG and with anti-CD3 antibody followed by Cy3-labeled goat antimouse IgG. (a) Donor cells, staining with anti-CD3 (red) and control rat IgG2a followed by secondary FITC-labeled goat antirat IgG (no signal). (b) Donor cells stained for CD3 (red) and FL (green). (c) Patient’s cells (M.T.) stained for CD3 (red) and FL (green). CD3 staining of patient’s cells was partly destroyed during handling due to drug-related fragility of cell membranes. Areas of overlap are highlighted in yellow.