While we and others have shown that anti-LPSFt IgM produced by B1 cells are protective [38,39], it has also been shown that Btk-/- mice, which lack B1a cells and IgM/IgG3, are more resistant to Ft infection [54] suggesting that B1a cells can have deleterious effect in tularemia. This evidence concerns the gene IGHG3 and tularemia.