IL21 and thanatophoric dysplasia: Collectively, our findings that IL‐21 increased the speed of passage through and frequency of entry into the cell cycle and promoted GC B cell accumulation over a large range of BCR affinities together with its previously reported roles in maintaining GC (Linterman et al, 2010; Zotos et al, 2010) and the LZ/DZ ratio (Collins & Speck, 2015; Zotos et al, 2021) are all indicative of the key role of IL‐21 in the initiation of a TD immune response being to promote the proliferation of pre‐GC and GC B cells.