Cross-linking induced patching and capping of CD15. MM6 cells (1 × 10⁶) were suspended in culture medium containing 10 μg FITC-conjugated anti-CD15 or anti-CD31 MoAb per mL and incubated at 4°C for 20 minutes. Cells were then washed and incubated with F(ab) ′₂ fragments of goat anti-mouse IgM (for CD15) or IgG (for CD31) at a concentration of 10 μg/mL at 4°C for 20 minutes. Cells were washed and then warmed to 37°C for 10 minutes and examined by fluorescent microscopy. Shown are phase-fluoresecence pairs (A and B, respectively) for cells incubated with anti-CD15 alone (second row), anti-CD15 plus F(ab) ′₂ secondary antibody (third row), and anti-CD31 plus F(ab) ′₂ secondary antibody (bottom). The top row shows cells incubated with the F(ab) ′₂ secondary antibody in the absence of primary antibody.