This study reports the cloning and characterization of human interleukin-17 (hIL-17), a cytokine produced by activated memory CD4+ T cells. hIL-17 is a 155-amino acid glycoprotein secreted as a homodimer. Unlike its viral counterpart, hIL-17 does not directly affect hematopoietic cells but stimulates stromal cells to produce proinflammatory and hematopoietic cytokines such as IL-6, IL-8, granulocyte-colony-stimulating factor (G-CSF), and prostaglandin E2 (PGE2). hIL-17 also supports the proliferation and maturation of CD34+ hematopoietic progenitors into neutrophils. These findings suggest that hIL-17 may play a role in initiating T cell-dependent inflammatory reactions and in the cytokine network that links the immune system to hematopoiesis. The study provides insights into the biological activities and potential roles of hIL-17 in immune responses and hematopoiesis.This study reports the cloning and characterization of human interleukin-17 (hIL-17), a cytokine produced by activated memory CD4+ T cells. hIL-17 is a 155-amino acid glycoprotein secreted as a homodimer. Unlike its viral counterpart, hIL-17 does not directly affect hematopoietic cells but stimulates stromal cells to produce proinflammatory and hematopoietic cytokines such as IL-6, IL-8, granulocyte-colony-stimulating factor (G-CSF), and prostaglandin E2 (PGE2). hIL-17 also supports the proliferation and maturation of CD34+ hematopoietic progenitors into neutrophils. These findings suggest that hIL-17 may play a role in initiating T cell-dependent inflammatory reactions and in the cytokine network that links the immune system to hematopoiesis. The study provides insights into the biological activities and potential roles of hIL-17 in immune responses and hematopoiesis.