TLRs primarily have two signaling pathways: the TRIF-dependent pathway (TLRs 3 and 4) and the MyD88-dependent pathway (TLRs 1, 2, 4, 5, 6, 7, 8, and 9). The most common signaling pathway that initiates immune responses is the MyD88-dependent pathway. MyD88 interacts with IRAK4, which then transmits the signal to NF-κB and MAPK in response to TLR activation. TLRs activate inflammatory cytokine expression through a MyD88-dependent pathway. TLRs on endosomes transmit signals through a TRIF-dependent pathway. TRIF, TRAF6, and IRAK1/2/4 collectively activate TAK1, or TRIF with the non-classical TBK1 and IKKε activate IRF3/7, which subsequently induce inflammatory cytokines.