SH2D1A and infection: To determine whether spontaneous GCs depend on T-cell help just like conventional GCs induced following immunization or infection, we examined wild-type (WT) B6 mice and mice deficient in SLAM-associated protein (SAP) that is essential for cognate T–B interactions and GC formation.22,23 As shown in Supplementary information, Fig. S3a, 3- to 4-month-old WT mice had ~0.5% of total B cells as GCs, while SAP-deficient mice had essentially none.