The discovery of T-helper 17 (Th17) cells, a unique CD4+ T-cell subset characterized by production of interleukin-17 (IL-17), has revolutionized our understanding of the role of T cells in human disease. Th17 cells are involved in the pathogenesis of a diverse group of immune-mediated diseases, including psoriasis, rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease, and asthma. They also play a role in tumorigenesis and transplant rejection. The biology of Th17 cells is complex and tightly regulated, with key signals including transforming growth factor-β (TGF-β) and interleukin-6 (IL-6). IL-23, while important in mouse Th17 differentiation, is less significant in humans. The transcription factor RORγt is crucial for Th17 differentiation, and STAT3 and STAT4 also play important roles. Th17 cells produce various inflammatory cytokines, such as IL-17A, IL-17F, and IL-22, which contribute to disease pathogenesis. They also express chemokine receptors like CCR6, which mediate their trafficking to inflamed tissues. In rheumatic diseases, Th17 cells are implicated in the pathogenesis of psoriasis, rheumatoid arthritis, and multiple sclerosis. Targeting IL-17 with neutralizing antibodies or soluble receptors is a promising therapeutic approach, but the safety of such treatments must be carefully considered due to the role of Th17 cells in normal host defenses against infection.The discovery of T-helper 17 (Th17) cells, a unique CD4+ T-cell subset characterized by production of interleukin-17 (IL-17), has revolutionized our understanding of the role of T cells in human disease. Th17 cells are involved in the pathogenesis of a diverse group of immune-mediated diseases, including psoriasis, rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease, and asthma. They also play a role in tumorigenesis and transplant rejection. The biology of Th17 cells is complex and tightly regulated, with key signals including transforming growth factor-β (TGF-β) and interleukin-6 (IL-6). IL-23, while important in mouse Th17 differentiation, is less significant in humans. The transcription factor RORγt is crucial for Th17 differentiation, and STAT3 and STAT4 also play important roles. Th17 cells produce various inflammatory cytokines, such as IL-17A, IL-17F, and IL-22, which contribute to disease pathogenesis. They also express chemokine receptors like CCR6, which mediate their trafficking to inflamed tissues. In rheumatic diseases, Th17 cells are implicated in the pathogenesis of psoriasis, rheumatoid arthritis, and multiple sclerosis. Targeting IL-17 with neutralizing antibodies or soluble receptors is a promising therapeutic approach, but the safety of such treatments must be carefully considered due to the role of Th17 cells in normal host defenses against infection.