T_FHdevelopment in healthy children and adults and in patients post-HSCT. Percentage of T_FH (the percentage of CD4⁺ T cells that are CD45RA⁻ and CXCR5⁺) was measured in (A) umbilical cord blood (UCB), healthy children of the ages shown, and healthy adults; (B) patients with IL2RG/JAK3 SCID at varying times post-HSCT; and patients with IL2RG/JAK3 SCID analyzed according to (C) conditioning, (D) donor B-cell chimerism, (E) Ig replacement status, and (F) response to tetanus vaccine. Medians and interquartile ranges are shown.