Increased ICF B-cell mortality after in vitro activation. Analysis of apoptotic and cycling cells from a healthy child (HD), P1, and P4 was done by propidium iodide staining on freshly isolated CD19⁺CD27^- naive B cells (time 0) or after 4 days' stimulation with rshCD40L and IL-4. Numbers in histograms indicate the percentage of apoptotic (sub-G₀) and cycling (S and G₂/M) cells analyzed on the FL2 wavelength on a FACScan flow cytometer after cell aggregates were gated out. Data shown for HD are representative of 5 healthy children.