Recent studies have shown that TLRs play a critical role in determining the fate of naive T cells, directing them toward either Th1 or Th2 responses (Dabbagh and Lewis, 2003; Xu et al., 2004) . To characterize the role of TLR signaling in determining a Th1 or Th2 response, IgG 1 (Th2) and IgG 2a (Th1) levels and the IgG 1 /IgG 2a ratio are calculated following Brucella aerosol exposure. IgG 1 and IgG 2a production are above background levels only 8 weeks p.i. (Figure 5 , panels D and E). By week 10, TLR2 and C57BL/6 mice produce significantly elevated levels of IgG 1 (Figure 5 , panel D) indicating TLR2 is not required for IgG 1 production. In contrast, IgG 1 levels in TLR4 and MyD88 knockout mice are significantly reduced, indicating TLR4 and MyD88 are essential for IgG 1 production (Figure 5, panel D) . Relatively low levels of IgG 2a are present at 10 weeks p.i. in all groups, but IgG 2a levels in TLR2 and MyD88 knockout mice are significantly reduced relative to control mice. IgG 1 /IgG 2a 