Transmembrane activator and calcium-modulator and cytophilin ligand interactor (TACI) is a TNFR family member molecule and is part of the so-called BAFF system. This includes two ligands: B cell-activating factor of the TNF family (BAFF) and a proliferation-inducing ligand (APRIL), and three receptors: BAFFR (TNFRSF13C), TACI (TNFRSF13B), and B cell maturation Ag (BCMA or TNFRSF17). BAFF binds the three receptors, while APRIL binds TACI and BCMA. TACI is constitutively expressed on na?ve B cells and is upregulated on activated B cells. TACI levels are severely reduced in newborn mice B cells compared with those of adult mice, and newborn B cells do not secret Igs when they are stimulated with BAFF or APRIL. This reduced TACI expression might be responsible for the irresponsiveness of newborn B cells to T cell independent type II antigens such as bacterial capsular polysaccharides.