B cells interactions with Tfh cells are critical in GC formation and propagation as (1) inborn errors in the CD40 receptor expressed on GC B cells or the CD40 ligand (CD40L) expressed on Tfh cells cause PID with complete lack of GCs and (2) CD40/CD40L blocking antibodies are sufficient to stop an ongoing GC reaction (46, 47). The gene discussed is CD40LG; the disease is pelvic inflammatory disease.