Reversible antigen-driven modulation of sIgM in B cells. (A) Oppositional changes in μ chain glycoforms mediated either by antigen (toward immature form) or by re-expression in vitro (toward mature form). (B) Hypothetical model of the effect of antigen on sIgM of B cells in vivo, suggesting that persistent exposure to antigen, followed by endocytosis, drives expression of immature glycan.