SIGLEC6 and immunodeficiency disease: FCRL4+ MBCs displayed “exhausted” or “atypical” MBC phenotypes, as observed in chronic inflammatory or immunodeficiency conditions [48–50], distinguished by high expression of TBX21 (encoding T-bet), B cell inhibitory receptor genes (FCRL4 and SIGLEC6 [49]), and homing receptor genes (CXCR3 and ITGAX [48, 49]) as well as low expression of CR2 (encoding CD21) and CD27 (Fig. S12D; Table S12).