Like all MAPK, p38 is activated by a kinase cascade resulting in dual phosphorylation of the activation loop (Thr-180 and Tyr-182). T cells possess an additional pathway downstream of the T cell receptor (TCR), in which p38 is phosphorylated on Tyr-323 by ZAP70, leading to auto-monophosphorylation of Thr-180 (alternative pathway). We have examined the physiologic role of the alternative pathway by creating knockin mice in which p38α and p38β Tyr-323 is replaced with a Phe and thus cannot be phosphorylated by ZAP70 (DKI mice). DKI mice are resistant to diseases such as experimental autoimmune encephalomyelitis (EAE) and collagen-induced arthritis. Analysis of transcription factor expression revealed that NF-ATc1 (NFAT2) and IRF-4, which are upstream of RORC and IL-17, are induced in CD4+T cells by TCR stimulation but not PMA and ionomycin, a stimulus that activates the classic MAPK cascade and is widely used to mimic TCR signaling. Consistent with this, induction of these proteins was markedly reduced in DKI CD4+T cells that lack the alternative pathway. Notably, activation of the classic p38 MAPK cascade by stress (UV irradiation or osmotic shock) abrogated TCR-mediated upregulation of NF-ATc1, IRF-4, and IL-17. When introduced in the gut of wild type mice, C. rodentium (citrobacter) induce an IL-17-dependent immune response that results in clearance of the pathogen. DKI mice, however, failed to mount an IL-17 response and did not eradicate the bacteria. Thus, TCR-mediated monophosphorylation of p38 is essential for IL-17 production, which is inhibited by its dual phosphorylation via the MAPK cascade.