Systemic sclerosis is characterized by extensive fibrosis, microvascular stenosis and autoantibody production. All three characteristics can be accounted for by activation of cells of the immune system. Activation of T cells is antigen-driven and occurs early in the course of the disease, before microscopic evidence of fibrosis. Activated T cells are predominantly of the type 2 T-helper lineage, and produce interleukin-4 and interleukin-13, which induce fibrosis. B cells are also activated early in the course of the disease and, through the production of autoantibodies, cause fibroblasts to adopt a profibrotic phenotype. Macrophages in perivascular infiltrates are activated and produce CC-chemokine ligand 2, transforming growth factor beta and platelet derived growth factor, all of which promote fibrosis and fibroproliferation. These new insights have direct impact on the treatment of patients with systemic sclerosis; therapies that target T cells, B cells and their harmful mediators are a logical approach, and preliminary data are promising.