IL21 and viral infectious disease: Furthermore, a B-cell stimulation protocol such as anti-CD40 + IL-4 + IL-21 that mimics physiologic B-cell stimulation through helper T cells, rather than reliance on mitogens or TLR agonists, enables selective activation of memory B cells [37] and may offer a superior strategy for identifying antigen-specific IgM+ memory B cells following resolution of viral infections such as COVID-19.