The primary function of neutrophil-derived serine proteases, neutrophil elastase, cathepsin G, and proteinase 3 (PR3) are thought to be the degradation of extracellular proteins at sites of inflammation, but excessive, prolonged, or inappropriate proteolytic activity causes harmful effects in the body. Although there are strong structural similarities among these proteases, PR3 has unique properties in many respects. In particular, PR3 is a major target antigen of autoantibodies, anti-neutrophil cytoplasmic antibodies (ANCA). Recent findings also revealed that PR3 is critically involved in the regulation of immune function. This review discusses the expression of PR3 in non-hemopoietic cells and focuses on the immune functions of PR3 with respect to direct modulation of cell signaling by PR3, PR3-mediated cell activation, and the possible involvement of protease-activated receptors, modulation of the cytokine network, innate immunity and PR3, and recent findings about the generation and function of ANCA.